Absolute space and time is a concept in physics and philosophy about the properties of the universe. In physics, absolute space and time may be a preferred frame.
Absolute, true and mathematical time, of itself, and from
its own nature flows equably without regard to anything external, and
by another name is called duration: relative, apparent and common time,
is some sensible and external (whether accurate or unequable) measure of
duration by the means of motion, which is commonly used instead of true
time ...
According to Newton, absolute time exists independently of any
perceiver and progresses at a consistent pace throughout the universe.
Unlike relative time, Newton believed absolute time was imperceptible
and could only be understood mathematically. According to Newton, humans
are only capable of perceiving relative time, which is a measurement of
perceivable objects in motion (like the Moon or Sun). From these
movements, we infer the passage of time.
Absolute space, in its own nature,
without regard to anything external, remains always similar and
immovable. Relative space is some movable dimension or measure of the
absolute spaces; which our senses determine by its position to bodies:
and which is vulgarly taken for immovable space ...
Absolute motion is the translation of a body from one absolute place
into another: and relative motion, the translation from one relative
place into another ...
— Isaac Newton
These notions imply that absolute space and time do not depend upon
physical events, but are a backdrop or stage setting within which
physical phenomena occur. Thus, every object has an absolute state of
motion relative to absolute space, so that an object must be either in a
state of absolute rest, or moving at some absolute speed.
To support his views, Newton provided some empirical examples:
according to Newton, a solitary rotating sphere can be inferred to
rotate about its axis relative to absolute space by observing the
bulging of its equator, and a solitary pair of spheres tied by a rope
can be inferred to be in absolute rotation about their center of gravity (barycenter) by observing the tension in the rope.
Differing views
Two
spheres orbiting around an axis. The spheres are distant enough for
their effects on each other to be ignored, and they are held together by
a rope. If the rope is under tension, it is because the bodies are
rotating relative to absolute space according to Newton, or because they rotate relative to the universe itself according to Mach, or because they rotate relative to local geodesics according to general relativity.
Historically, there have been differing views on the concept of absolute space and time. Gottfried Leibniz
was of the opinion that space made no sense except as the relative
location of bodies, and time made no sense except as the relative
movement of bodies. George Berkeley
suggested that, lacking any point of reference, a sphere in an
otherwise empty universe could not be conceived to rotate, and a pair of
spheres could be conceived to rotate relative to one another, but not
to rotate about their center of gravity, an example later raised by Albert Einstein in his development of general relativity.
A more recent form of these objections was made by Ernst Mach. Mach's principle proposes that mechanics is entirely about relative motion of bodies and, in particular, mass
is an expression of such relative motion. So, for example, a single
particle in a universe with no other bodies would have zero mass.
According to Mach, Newton's examples simply illustrate relative rotation
of spheres and the bulk of the universe.
When, accordingly, we say that a body preserves unchanged its direction and velocity in space, our assertion is nothing more or less than an abbreviated reference to the entire universe. —Ernst Mach
These views opposing absolute space and time may be seen from a modern stance as an attempt to introduce operational definitions for space and time, a perspective made explicit in the special theory of relativity.
Even within the context of Newtonian mechanics, the modern view is that absolute space is unnecessary. Instead, the notion of inertial frame of reference has taken precedence, that is, a preferred set of frames of reference that move uniformly with respect to one another. The laws of physics transform from one inertial frame to another according to Galilean relativity, leading to the following objections to absolute space, as outlined by Milutin Blagojević:
The existence of absolute space contradicts the internal logic
of classical mechanics since, according to Galilean principle of
relativity, none of the inertial frames can be singled out.
Absolute space does not explain inertial forces since they are
related to acceleration with respect to any one of the inertial frames.
Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon.
Newton himself recognized the role of inertial frames.
The motions of bodies included in a given space are the
same among themselves, whether that space is at rest or moves uniformly
forward in a straight line.
As a practical matter, inertial frames often are taken as frames moving uniformly with respect to the fixed stars. See Inertial frame of reference for more discussion on this.
Mathematical definitions
Space, as understood in Newtonian mechanics, is three-dimensional and Euclidean, with a fixed orientation. It is denoted E3. If some point O in E3 is fixed and defined as an origin, the position of any point P in E3 is uniquely determined by its radius vector (the origin of this vector coincides with the point O and its end coincides with the point P). The three-dimensional linear vector spaceR3 is a set of all radius vectors. The space R3 is endowed with a scalar product ⟨ , ⟩.
Time is a scalar which is the same in all space E3 and is denoted as t. The ordered set { t } is called a time axis.
The above four concepts are the "well-known" objects mentioned by Isaac Newton in his Principia:
I do not define time, space, place and motion, as being well known to all.
Special relativity
The concepts of space and time were separate in physical theory prior to the advent of special relativity
theory, which connected the two and showed both to be dependent upon
the reference frame's motion. In Einstein's theories, the ideas of
absolute time and space were superseded by the notion of spacetime in special relativity, and curved spacetime in general relativity.
Absolute simultaneity refers to the concurrence of events in time at different locations in space in a manner agreed upon in all frames of reference. The theory of relativity does not have a concept of absolute time because there is a relativity of simultaneity.
An event that is simultaneous with another event in one frame of
reference may be in the past or future of that event in a different
frame of reference, which negates absolute simultaneity.
Quoted below from his later papers, Einstein identified the term aether with "properties of space", a terminology that is not widely used. Einstein stated that in general relativity the "aether" is not absolute anymore, as the geodesic and therefore the structure of spacetime depends on the presence of matter.
To deny the ether is ultimately to
assume that empty space has no physical qualities whatever. The
fundamental facts of mechanics do not harmonize with this view. For the
mechanical behaviour of a corporeal system hovering freely in empty space
depends not only on relative positions (distances) and relative
velocities, but also on its state of rotation, which physically may be
taken as a characteristic not appertaining to the system in itself. In
order to be able to look upon the rotation of the system, at least
formally, as something real, Newton objectivises space. Since he classes
his absolute space together with real things, for him rotation relative
to an absolute space is also something real. Newton might no less well
have called his absolute space “Ether”; what is essential is merely that
besides observable objects, another thing, which is not perceptible,
must be looked upon as real, to enable acceleration or rotation to be looked upon as something real.
— Albert Einstein, Ether and the Theory of Relativity (1920)
Because it was no longer possible
to speak, in any absolute sense, of simultaneous states at different
locations in the aether, the aether became, as it were,
four-dimensional, since there was no objective way of ordering its
states by time alone. According to special relativity too, the aether
was absolute, since its influence on inertia and the propagation of
light was thought of as being itself independent of physical
influence....The theory of relativity resolved this problem by
establishing the behaviour of the electrically neutral point-mass by the
law of the geodetic line, according to which inertial and gravitational
effects are no longer considered as separate. In doing so, it attached
characteristics to the aether which vary from point to point,
determining the metric and the dynamic behaviour of material points, and
determined, in their turn, by physical factors, namely the distribution
of mass/energy. Thus the aether of general relativity differs from
those of classical mechanics and special relativity in that it is not
‘absolute’ but determined, in its locally variable characteristics, by
ponderable matter.
— Albert Einstein, Über den Äther (1924)
General relativity
Special
relativity eliminates absolute time (although Gödel and others suspect
absolute time may be valid for some forms of general relativity) and general relativity further reduces the physical scope of absolute space and time through the concept of geodesics. There appears to be absolute space in relation to the distant stars
because the local geodesics eventually channel information from these
stars, but it is not necessary to invoke absolute space with respect to
any system's physics, as its local geodesics are sufficient to describe
its spacetime.
To send or receive files, users use a BitTorrent client on their Internet-connected computer, which are available for a variety of computing platforms and operating systems, including an official client. BitTorrent trackers
provide a list of files available for transfer and allow the client to
find peer users, known as "seeds", who may transfer the files.
BitTorrent downloading is considered to be faster than HTTP ("direct downloading") and FTP due to the lack of a central server that could limit bandwidth.
BitTorrent is one of the most common protocols for transferring large files, such as digital video files containing TV shows and video clips, or digital audio files. BitTorrent accounted for a third of all internet traffic in 2004, according to a study by Cachelogic. As recently as 2019 BitTorrent remained a significant file sharing protocol according to Sandvine, generating a substantial amount of Internet traffic, with 2.46% of downstream, and 27.58% of upstream traffic, although this share has declined significantly since then.
History
The middle computer is acting as a "seed" to provide a file to the other computers which act as peers.
Programmer Bram Cohen, a University at Buffalo alumnus, designed the protocol in April 2001, and released the first available version on 2 July 2001. Cohen and Ashwin Navin founded BitTorrent, Inc. (later renamed Rainberry, Inc.) to further develop the technology in 2004.
The first release of the BitTorrent client had no search engine
and no peer exchange. Up until 2005, the only way to share files was by
creating a small text file called a "torrent", that they would upload to a torrent index site. The first uploader acted as a seed, and downloaders would initially connect as peers.
Those who wish to download the file would download the torrent, which
their client would use to connect to a tracker which had a list of the
IP addresses of other seeds and peers in the swarm. Once a peer
completed a download of the complete file, it could in turn function as a
seed. These files contain metadata about the files to be shared and the trackers which keep track of the other seeds and peers.
In 2005, first Vuze
and then the BitTorrent client introduced distributed tracking using
distributed hash tables which allowed clients to exchange data on swarms
directly without the need for a torrent file.
In 2006, peer exchange functionality was added allowing clients to add peers based on the data found on connected nodes.
In 2017, BitTorrent, Inc. released the BitTorrent v2 protocol specification.
BitTorrent v2 is intended to work seamlessly with previous versions of
the BitTorrent protocol. The main reason for the update was that the old
cryptographic hash function, SHA-1, is no longer considered safe from malicious attacks by the developers, and as such, v2 uses SHA-256.
To ensure backwards compatibility, the v2 .torrent file format supports
a hybrid mode where the torrents are hashed through both the new method
and the old method, with the intent that the files will be shared with
peers on both v1 and v2 swarms. Another update to the specification is
adding a hash tree
to speed up time from adding a torrent to downloading files, and to
allow more granular checks for file corruption. In addition, each file
is now hashed individually, enabling files in the swarm to be
deduplicated, so that if multiple torrents include the same files, but
seeders are only seeding the file from some, downloaders of the other
torrents can still download the file. In addition, file hashes can be
displayed on tracker, torrent indexing services, to search for swarms by
searching for hashes of files contained in them. These hashes are
different from the usual SHA-256 hash of files and can be obtained using
tools. Magnet links for v2 also support a hybrid mode to ensure support for legacy clients.
Design
Animation of protocol use: The colored dots beneath each computer in the animation represent different parts of the file being shared.
By the time a copy to a destination computer of each of those parts
completes, a copy to another destination computer of that part (or other
parts) is already taking place between users.
The BitTorrent protocol can be used to reduce the server and network
impact of distributing large files. Rather than downloading a file from a
single source server, the BitTorrent protocol allows users to join a
"swarm" of hosts to upload and download from each other simultaneously.
The protocol is an alternative to the older single source, multiple
mirror sources technique for distributing data, and can work effectively
over networks with lower bandwidth.
Using the BitTorrent protocol, several basic computers, such as home
computers, can replace large servers while efficiently distributing
files to many recipients. This lower bandwidth usage also helps prevent
large spikes in internet traffic
in a given area, keeping internet speeds higher for all users in
general, regardless of whether or not they use the BitTorrent protocol.
The file being distributed is divided into segments called pieces.
As each peer receives a new piece of the file, it becomes a source (of
that piece) for other peers, relieving the original seed from having to
send that piece to every computer or user wishing a copy. With
BitTorrent, the task of distributing the file is shared by those who
want it; it is entirely possible for the seed to send only a single copy
of the file itself and eventually distribute to an unlimited number of
peers. Each piece is protected by a cryptographic hash contained in the torrent descriptor.
This ensures that any modification of the piece can be reliably
detected, and thus prevents both accidental and malicious modifications
of any of the pieces received at other nodes. If a node starts with an
authentic copy of the torrent descriptor, it can verify the authenticity
of the entire file it receives.
Pieces are typically downloaded non-sequentially, and are
rearranged into the correct order by the BitTorrent client, which
monitors which pieces it needs, and which pieces it has and can upload
to other peers. Pieces are of the same size throughout a single download
(for example, a 10 MB file may be transmitted as ten 1 MB pieces or as
forty 256 KB pieces).
Due to the nature of this approach, the download of any file can be
halted at any time and be resumed at a later date, without the loss of
previously downloaded information, which in turn makes BitTorrent
particularly useful in the transfer of larger files. This also enables
the client to seek out readily available pieces and download them
immediately, rather than halting the download and waiting for the next
(and possibly unavailable) piece in line, which typically reduces the
overall time of the download. This eventual transition from peers to
seeders determines the overall "health" of the file (as determined by
the number of times a file is available in its complete form).
The distributed nature of BitTorrent can lead to a flood-like
spreading of a file throughout many peer computer nodes. As more peers
join the swarm, the likelihood of a successful download by any
particular node increases. Relative to traditional Internet distribution
schemes, this permits a significant reduction in the original
distributor's hardware and bandwidth resource costs. Distributed
downloading protocols in general provide redundancy against system problems, reduce dependence on the original distributor, and provide sources for the file which are generally transient and therefore there is no single point of failure as in one way server-client transfers.
Though both ultimately transfer files over a network, a
BitTorrent download differs from a one way server-client download (as is
typical with an HTTP or FTP request, for example) in several fundamental ways:
BitTorrent makes many small data requests over different IP connections to different machines, while server-client downloading is typically made via a single TCP connection to a single machine.
BitTorrent downloads in a random or in a "rarest-first" approach that ensures high availability, while classic downloads are sequential.
Taken together, these differences allow BitTorrent to achieve much
lower cost to the content provider, much higher redundancy, and much
greater resistance to abuse or to "flash crowds" than regular server software.
However, this protection, theoretically, comes at a cost: downloads can
take time to rise to full speed because it may take time for enough
peer connections to be established, and it may take time for a node to
receive sufficient data to become an effective uploader. This contrasts
with regular downloads (such as from an HTTP server, for example) that,
while more vulnerable to overload and abuse, rise to full speed very
quickly, and maintain this speed throughout. In the beginning,
BitTorrent's non-contiguous download methods made it harder to support
"streaming playback". In 2014, the client Popcorn Time allowed for streaming of BitTorrent video files. Since then, more and more clients are offering streaming options.
Searching
The
BitTorrent protocol provides no way to index torrent files. As a
result, a comparatively small number of websites have hosted a large
majority of torrents, many linking to copyrighted works without the
authorization of copyright holders, rendering those sites especially
vulnerable to lawsuits. A BitTorrent index is a "list of .torrent files, which typically includes descriptions" and information about the torrent's content.
Several types of websites support the discovery and distribution of
data on the BitTorrent network. Public torrent-hosting sites such as The Pirate Bay
allow users to search and download from their collection of torrent
files. Users can typically also upload torrent files for content they
wish to distribute. Often, these sites also run BitTorrent trackers
for their hosted torrent files, but these two functions are not
mutually dependent: a torrent file could be hosted on one site and
tracked by another unrelated site. Private host/tracker sites operate
like public ones except that they may restrict access to registered
users and may also keep track of the amount of data each user uploads
and downloads, in an attempt to reduce "leeching".
Web search engines allow the discovery of torrent files that are hosted and tracked on other sites; examples include The Pirate Bay and BTDigg.
These sites allow the user to ask for content meeting specific criteria
(such as containing a given word or phrase) and retrieve a list of
links to torrent files matching those criteria. This list can often be
sorted with respect to several criteria, relevance (seeders to leechers
ratio) being one of the most popular and useful (due to the way the
protocol behaves, the download bandwidth achievable is very sensitive to
this value). Metasearch engines allow one to search several BitTorrent indices and search engines at once.
The Tribler
BitTorrent client was among the first to incorporate built-in search
capabilities. With Tribler, users can find .torrent files held by random
peers and taste buddies. It adds such an ability to the BitTorrent protocol using a gossip protocol, somewhat similar to the eXeem
network which was shut down in 2005. The software includes the ability
to recommend content as well. After a dozen downloads, the Tribler
software can roughly estimate the download taste of the user, and
recommend additional content.
In May 2007, researchers at Cornell University published a paper proposing a new approach to searching a peer-to-peer network for inexact strings,
which could replace the functionality of a central indexing site. A
year later, the same team implemented the system as a plugin for Vuze called Cubit and published a follow-up paper reporting its success.
A somewhat similar facility but with a slightly different approach is provided by the BitComet client through its "Torrent Exchange"
feature. Whenever two peers using BitComet (with Torrent Exchange
enabled) connect to each other they exchange lists of all the torrents
(name and info-hash) they have in the Torrent Share storage (torrent
files which were previously downloaded and for which the user chose to
enable sharing by Torrent Exchange). Thus each client builds up a list
of all the torrents shared by the peers it connected to in the current
session (or it can even maintain the list between sessions if
instructed).
At any time the user can search into that Torrent Collection list
for a certain torrent and sort the list by categories. When the user
chooses to download a torrent from that list, the .torrent file is
automatically searched for (by info-hash value) in the DHT Network and when found it is downloaded by the querying client which can subsequently create and initiate a downloading task.
Downloading and sharing
Users
find a torrent of interest on a torrent index site or by using a search
engine built into the client, download it, and open it with a
BitTorrent client. The client connects to the tracker(s) or seeds
specified in the torrent file, from which it receives a list of seeds
and peers currently transferring pieces of the file(s). The client
connects to those peers to obtain the various pieces. If the swarm
contains only the initial seeder, the client connects directly to it,
and begins to request pieces. Clients incorporate mechanisms to optimize
their download and upload rates.
The effectiveness of this data exchange depends largely on the
policies that clients use to determine to whom to send data. Clients may
prefer to send data to peers that send data back to them (a "tit for tat"
exchange scheme), which encourages fair trading. But strict policies
often result in suboptimal situations, such as when newly joined peers
are unable to receive any data because they do not have any pieces yet
to trade themselves or when two peers with a good connection between
them do not exchange data simply because neither of them takes the
initiative. To counter these effects, the official BitTorrent client
program uses a mechanism called "optimistic unchoking", whereby the
client reserves a portion of its available bandwidth for sending pieces
to random peers (not necessarily known good partners, or "preferred
peers") in hopes of discovering even better partners and to ensure that
newcomers get a chance to join the swarm.
Although "swarming" scales well to tolerate "flash crowds" for popular content, it is less useful for unpopular or niche market
content. Peers arriving after the initial rush might find the content
unavailable and need to wait for the arrival of a "seed" in order to
complete their downloads. The seed arrival, in turn, may take long to
happen (this is termed the "seeder promotion problem"). Since
maintaining seeds for unpopular content entails high bandwidth and
administrative costs, this runs counter to the goals of publishers that
value BitTorrent as a cheap alternative to a client-server approach.
This occurs on a huge scale; measurements have shown that 38% of all new
torrents become unavailable within the first month.
A strategy adopted by many publishers which significantly increases
availability of unpopular content consists of bundling multiple files in
a single swarm.
More sophisticated solutions have also been proposed; generally, these
use cross-torrent mechanisms through which multiple torrents can
cooperate to better share content.
Creating and publishing
The peer distributing a data file treats the file as a number of
identically sized pieces, usually with byte sizes of a power of 2, and
typically between 32 KB and 16 MB each. The peer creates a hash for each piece, using the SHA-1
hash function, and records it in the torrent file. Pieces with sizes
greater than 512 KB will reduce the size of a torrent file for a very
large payload, but is claimed to reduce the efficiency of the protocol.
When another peer later receives a particular piece, the hash of the
piece is compared to the recorded hash to test that the piece is
error-free.
Peers that provide a complete file are called seeders, and the peer
providing the initial copy is called the initial seeder. The exact
information contained in the torrent file depends on the version of the
BitTorrent protocol.
By convention, the name of a torrent file has the suffix .torrent. Torrent files use the Bencode file format, and contain an "announce" section, which specifies the URL of the tracker, and an "info" section, containing (suggested) names for the files, their lengths, the piece length used, and a SHA-1hash code
for each piece, all of which are used by clients to verify the
integrity of the data they receive. Though SHA-1 has shown signs of
cryptographic weakness, Bram Cohen did not initially consider the risk
big enough for a backward incompatible change to, for example, SHA-3. As of BitTorrent v2 the hash function has been updated to SHA-256.
In the early days, torrent files were typically published to
torrent index websites, and registered with at least one tracker. The
tracker maintained lists of the clients currently connected to the
swarm. Alternatively, in a trackerless system (decentralized tracking) every peer acts as a tracker. Azureus was the first BitTorrent client to implement such a system through the distributed hash table (DHT) method. An alternative and incompatible DHT system, known as Mainline DHT, was released in the Mainline BitTorrent client three weeks later (though it had been in development since 2002) and subsequently adopted by the μTorrent, Transmission, rTorrent, KTorrent, BitComet, and Deluge clients.
After the DHT was adopted, a "private" flag – analogous to the broadcast flag – was unofficially introduced, telling clients to restrict the use of decentralized tracking regardless of the user's desires.
The flag is intentionally placed in the info section of the torrent so
that it cannot be disabled or removed without changing the identity of
the torrent. The purpose of the flag is to prevent torrents from being
shared with clients that do not have access to the tracker. The flag was
requested for inclusion in the official specification in August 2008,
but has not been accepted yet. Clients that have ignored the private flag were banned by many trackers, discouraging the practice.
Anonymity
BitTorrent does not, on its own, offer its users anonymity. One can usually see the IP addresses of all peers in a swarm in one's own client or firewall program. This may expose users with insecure systems to attacks. In some countries, copyright organizations scrape lists of peers, and send takedown notices to the internet service provider
of users participating in the swarms of files that are under copyright.
In some jurisdictions, copyright holders may launch lawsuits against
uploaders or downloaders for infringement, and police may arrest
suspects in such cases.
Various means have been used to promote anonymity. For example, the BitTorrent client Tribler makes available a Tor-like onion network,
optionally routing transfers through other peers to obscure which
client has requested the data. The exit node would be visible to peers
in a swarm, but the Tribler organization provides exit nodes. One
advantage of Tribler is that clearnet torrents can be downloaded with only a small decrease in download speed from one "hop" of routing.
i2p
provides a similar anonymity layer although in that case, one can only
download torrents that have been uploaded to the i2p network. The bittorrent client Vuze allows users who are not concerned about anonymity to take clearnet torrents, and make them available on the i2p network.
Most BitTorrent clients are not designed to provide anonymity when used over Tor, and there is some debate as to whether torrenting over Tor acts as a drag on the network.
Private torrent trackers are usually invitation only, and require
members to participate in uploading, but have the downside of a single
centralized point of failure. Oink's Pink Palace and What.cd are examples of private trackers which have been shut down.
Seedbox services download the torrent files first to the company's servers, allowing the user to direct download the file from there. One's IP address would be visible to the Seedbox provider, but not to third parties.
Virtual private networks
encrypt transfers, and substitute a different IP address for the
user's, so that anyone monitoring a torrent swarm will only see that
address.
Associated technologies
Distributed trackers
On 2 May 2005, Azureus 2.3.0.0 (now known as Vuze) was released, utilizing a distributed database system. This system is a distributed hash table implementation which allows the client to use torrents that do not have a working BitTorrent tracker.
A bootstrap server is instead utilized. The following month,
BitTorrent, Inc. released version 4.2.0 of the Mainline BitTorrent
client, which supported an alternative DHT implementation (popularly
known as "Mainline DHT",
outlined in a draft on their website) that is incompatible with that of
Azureus. In 2014, measurement showed concurrent users of Mainline DHT
to be from 10 million to 25 million, with a daily churn of at least
10 million.
Current versions of the official BitTorrent client, μTorrent, BitComet, Transmission and BitSpirit all share compatibility with Mainline DHT. Both DHT implementations are based on Kademlia.
As of version 3.0.5.0, Azureus also supports Mainline DHT in addition
to its own distributed database through use of an optional application
plugin. This potentially allows the Azureus/Vuze client to reach a bigger swarm.
Another idea that has surfaced in Vuze is that of virtual torrents. This idea is based on the distributed tracker approach and is used to describe some web resource. Currently, it is used for instant messaging.
It is implemented using a special messaging protocol and requires an
appropriate plugin. Anatomic P2P is another approach, which uses a
decentralized network of nodes that route traffic to dynamic trackers.
Most BitTorrent clients also use peer exchange (PEX) to gather peers in addition to trackers and DHT.
Peer exchange checks with known peers to see if they know of any other
peers. With the 3.0.5.0 release of Vuze, all major BitTorrent clients
now have compatible peer exchange.
Web "seeding" was implemented in 2006 as the ability of BitTorrent
clients to download torrent pieces from an HTTP source in addition to
the "swarm". The advantage of this feature is that a website may
distribute a torrent for a particular file or batch of files and make
those files available for download from that same web server; this can
simplify long-term seeding and load balancing
through the use of existing, cheap, web hosting setups. In theory, this
would make using BitTorrent almost as easy for a web publisher as
creating a direct HTTP download. In addition, it would allow the "web
seed" to be disabled if the swarm becomes too popular while still
allowing the file to be readily available. This feature has two distinct
specifications, both of which are supported by Libtorrent and the 26+ clients that use it.
The first was created by John "TheSHAD0W" Hoffman, who created BitTornado.
This first specification requires running a web service that serves
content by info-hash and piece number, rather than filename.
The other specification is created by GetRight authors and can rely on a basic HTTP download space (using byte serving).
In September 2010, a new service named Burnbit was launched which generates a torrent from any URL using webseeding.
There are server-side solutions that provide initial seeding of the
file from the web server via standard BitTorrent protocol and when the
number of external seeders reach a limit, they stop serving the file
from the original source.
A technique called broadcatching combines RSS
feeds with the BitTorrent protocol to create a content delivery system,
further simplifying and automating content distribution. Steve Gillmor
explained the concept in a column for Ziff-Davis in December 2003. The discussion spread quickly among bloggers (Ernest Miller, Chris Pirillo, etc.). In an article entitled Broadcatching with BitTorrent, Scott Raymond explained:
I want RSS feeds of BitTorrent
files. A script would periodically check the feed for new items, and use
them to start the download. Then, I could find a trusted publisher of
an Alias
RSS feed, and "subscribe" to all new episodes of the show, which would
then start downloading automatically – like the "season pass" feature of
the TiVo.
— Scott Raymond, scottraymond.net
The RSS feed will track the content, while BitTorrent ensures content integrity with cryptographichashing of all data, so feed subscribers will receive uncorrupted content. One of the first and popular software clients (free and open source) for broadcatching is Miro.
Other free software clients such as PenguinTV and KatchTV are also now
supporting broadcatching. The BitTorrent web-service MoveDigital added
the ability to make torrents available to any web application capable of
parsing XML through its standard REST-based interface in 2006, though this has since been discontinued. Additionally, Torrenthut is developing a similar torrent API that will provide the same features, and help bring the torrent community to Web 2.0 standards. Alongside this release is a first PHP application built using the API called PEP, which will parse any Really Simple Syndication (RSS 2.0) feed and automatically create and seed a torrent for each enclosure found in that feed.
Since BitTorrent makes up a large proportion of total traffic, some ISPs
have chosen to "throttle" (slow down) BitTorrent transfers. For this
reason, methods have been developed to disguise BitTorrent traffic in an
attempt to thwart these efforts. Protocol header encrypt (PHE) and Message stream encryption/Protocol encryption (MSE/PE) are features of some BitTorrent clients that attempt to make BitTorrent hard to detect and throttle. As of November 2015, Vuze, BitComet, KTorrent, Transmission, Deluge, μTorrent, MooPolice, Halite, qBittorrent, rTorrent, and the latest official BitTorrent client (v6) support MSE/PE encryption.
In August 2007, Comcast
was preventing BitTorrent seeding by monitoring and interfering with
the communication between peers. Protection against these efforts is
provided by proxying the client-tracker traffic via an encrypted tunnel to a point outside of the Comcast network. In 2008, Comcast called a "truce" with BitTorrent, Inc. with the intention of shaping traffic in a protocol-agnostic manner. Questions about the ethics and legality of Comcast's behavior have led to renewed debate about net neutrality in the United States. In general, although encryption can make it difficult to determine what is being shared, BitTorrent is vulnerable to traffic analysis.
Thus, even with MSE/PE, it may be possible for an ISP to recognize
BitTorrent and also to determine that a system is no longer downloading
but only uploading data, and terminate its connection by injecting TCP RST (reset flag) packets.
Multitrackers
Another unofficial feature is an extension to the BitTorrent metadata format proposed by John Hoffman
and implemented by several indexing websites. It allows the use of
multiple trackers per file, so if one tracker fails, others can continue
to support file transfer. It is implemented in several clients, such as
BitComet, BitTornado, BitTorrent, KTorrent, Transmission, Deluge, μTorrent, rtorrent, Vuze, and Frostwire.
Trackers are placed in groups, or tiers, with a tracker randomly chosen
from the top tier and tried, moving to the next tier if all the
trackers in the top tier fail.
Torrents with multiple trackers can decrease the time it takes to download a file, but also have a few consequences:
Poorly implemented clients may contact multiple trackers, leading to more overhead-traffic.
Torrents from closed trackers suddenly become downloadable by non-members, as they can connect to a seed via an open tracker.
Peer selection
As of December 2008,
BitTorrent, Inc. was working with Oversi on new Policy Discover
Protocols that query the ISP for capabilities and network architecture
information. Oversi's ISP hosted NetEnhancer box is designed to "improve
peer selection" by helping peers find local nodes, improving download
speeds while reducing the loads into and out of the ISP's network.
Some BitTorrent implementations such as MLDonkey
and Torrentflux are designed to run as servers. For example, this can
be used to centralize file sharing on a single dedicated server which
users share access to on the network. Server-oriented BitTorrent implementations can also be hosted by hosting providers at co-located
facilities with high bandwidth Internet connectivity (e.g., a
datacenter) which can provide dramatic speed benefits over using
BitTorrent from a regular home broadband connection. Services such as ImageShack can download files on BitTorrent for the user, allowing them to download the entire file by HTTP once it is finished.
The Operaweb browser supports BitTorrent natively. Brave web browser ships with an extension which supports WebTorrent, a BitTorrent-like protocol based on WebRTC instead of UDP and TCP.BitLet allowed users to download Torrents directly from their browser using a Java applet (until browsers removed support for Java applets).
An increasing number of hardware devices are being made to support
BitTorrent. These include routers and NAS devices containing
BitTorrent-capable firmware like OpenWrt. Proprietary versions of the protocol which implement DRM, encryption, and authentication are found within managed clients such as Pando.
Adoption
A growing number of individuals and organizations are using BitTorrent to distribute their own or licensed works (e.g. indie
bands distributing digital files of their new songs). Independent
adopters report that BitTorrent technology reduces demands on private
networking hardware and bandwidth, an essential for non-profit groups
with large amounts of internet traffic.
Many major open source and free software projects encourage BitTorrent as well as conventional downloads of their products (via HTTP, FTP etc.) to increase availability and to reduce load on their own servers, especially when dealing with larger files.
In addition, some video game installers, especially those whose large
size makes them difficult to host due to bandwidth limits, extremely
frequent downloads, and unpredictable changes in network traffic, will
distribute instead a specialized, stripped down BitTorrent client with
enough functionality to download the game from the other running clients
and the primary server (which is maintained in case not enough peers
are available).
Some uses of BitTorrent for file sharing may violate laws in some jurisdictions (see legislation section).
Popularity and traffic statistics
As of January 2012,
BitTorrent is utilized by 150 million active users. Based on this
figure, the total number of monthly users may be estimated to more than a
quarter of a billion (≈ 250 million). As of February 2013, BitTorrent was responsible for 3.35% of all worldwide bandwidth—more than half of the 6% of total bandwidth dedicated to file sharing. As of 2013, BitTorrent had 15–27 million concurrent users at any time.
Film, video, and music
BitTorrent Inc. has obtained a number of licenses from Hollywood studios for distributing popular content from their websites.
Sub Pop Records releases tracks and videos via BitTorrent Inc. to distribute its 1000+ albums. Babyshambles and The Libertines (both bands associated with Pete Doherty) have extensively used torrents to distribute hundreds of demos and live videos. US industrial rock band Nine Inch Nails frequently distributes albums via BitTorrent.
Podcasting
software has integrated BitTorrent to help podcasters deal with the
download demands of their MP3 "radio" programs. Specifically, Juice and Miro (formerly known as Democracy Player) support automatic processing of .torrent files from RSS feeds. Similarly, some BitTorrent clients, such as μTorrent, are able to process web feeds and automatically download content found within them.
VODO, a service which distributes "free-to-share" movies and TV shows via BitTorrent.
Broadcasters
In 2008, the CBC became the first public broadcaster in North America to make a full show (Canada's Next Great Prime Minister) available for download using BitTorrent.
The Norwegian Broadcasting Corporation (NRK) has since March 2008 experimented with bittorrent distribution, available online.
Only selected works in which NRK owns all royalties are published.
Responses have been very positive, and NRK is planning to offer more
content.
The Dutch VPRO broadcasting organization released four documentaries in 2009 and 2010 under a Creative Commons license using the content distribution feature of the Mininova tracker.
Cloud Service Providers
The Amazon AWS's Simple Storage Service
(S3), until April 29, 2021, had supported sharing of bucket objects
with BitTorrent protocols. As of June 13, 2020, the feature is only
available in service regions launched after May 30, 2016. The feature for the existing customers will be extended for an
additional 12 months following the deprecation. After April 29, 2022,
BitTorrent clients will no longer connect to Amazon S3.
Florida State University uses BitTorrent to distribute large scientific data sets to its researchers.
Many universities that have BOINC
distributed computing projects have used the BitTorrent functionality
of the client-server system to reduce the bandwidth costs of
distributing the client-side applications used to process the scientific
data. If a BOINC distributed computing application needs to be updated (or merely sent to a user), it can do so with little impact on the BOINC server.
Academic Torrents is a BitTorrent tracker for use by researchers in fields that need to share large datasets.
Others
Facebook uses BitTorrent to distribute updates to Facebook servers.
Twitter uses BitTorrent to distribute updates to Twitter servers.
The Internet Archive
added BitTorrent to its file download options for over 1.3 million
existing files, and all newly uploaded files, in August 2012. This method is the fastest means of downloading media from the Archive.
By early 2015, AT&T estimated that BitTorrent accounted for 20% of all broadband traffic.
Routers that use network address translation
(NAT) must maintain tables of source and destination IP addresses and
ports. Because BitTorrent frequently contacts 20–30 servers per second,
the NAT tables of some consumer-grade routers are rapidly filled. This
is a known cause of some home routers ceasing to work correctly.
Although the protocol itself is legal,
problems stem from using the protocol to traffic copyright infringing
works, since BitTorrent is often used to download otherwise paid
content, such as movies and video games. There has been much controversy
over the use of BitTorrent trackers. BitTorrent metafiles themselves do
not store file contents. Whether the publishers of BitTorrent metafiles
violate copyrights by linking to copyrighted works without the
authorization of copyright holders is controversial. Various
jurisdictions have pursued legal action against websites that host
BitTorrent trackers.
High-profile examples include the closing of Suprnova.org, TorrentSpy, LokiTorrent, BTJunkie, Mininova, Oink's Pink Palace and What.cd. BitTorrent search engine The Pirate Bay
torrent website, formed by a Swedish group, is noted for the "legal"
section of its website in which letters and replies on the subject of
alleged copyright infringements are publicly displayed. On 31 May 2006,
The Pirate Bay's servers in Sweden were raided by Swedish police on
allegations by the MPAA of copyright infringement;
however, the tracker was up and running again three days later. In the
study used to value NBC Universal in its merger with Comcast, Envisional
examined the 10,000 torrent swarms managed by PublicBT which had the
most active downloaders. After excluding pornographic and unidentifiable
content, it was found that only one swarm offered legitimate content.
In the United States, more than 200,000 lawsuits have been filed for copyright infringement on BitTorrent since 2010. In the United Kingdom, on 30 April 2012, the High Court of Justice ordered five ISPs to block The Pirate Bay.
Security
One concern is the UDP flood attack. BitTorrent implementations often use μTP for their communication. To achieve high bandwidths, the underlying protocol used is UDP, which allows spoofing of source addresses of internet traffic. It has been possible to carry out denial-of-service attacks in a P2P lab environment, where users running BitTorrent clients act as amplifiers for an attack at another service. However this is not always an effective attack because ISPs can check if the source address is correct.
Several studies on BitTorrent found files available for download containing malware. In particular, one small sample indicated that 18% of all executable programs available for download contained malware. Another study claims that as much as 14.5% of BitTorrent downloads contain zero-day malware, and that BitTorrent was used as the distribution mechanism for 47% of all zero-day malware they have found.
https://en.wikipedia.org/wiki/Cosmology The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies
ever photographed at that time. Except for the few stars in the
foreground (which are bright and easily recognizable because only they
have diffraction spikes),
every speck of light in the photo is an individual galaxy, some of them
as old as 13.2 billion years; the observable universe is estimated to
contain more than 2 trillion galaxies.
Physics and astrophysics have played central roles in shaping our understanding of the universe through scientific observation and experiment. Physical cosmology
was shaped through both mathematics and observation in an analysis of
the whole universe. The universe is generally understood to have begun
with the Big Bang, followed almost instantaneously by cosmic inflation, an expansion of space from which the universe is thought to have emerged 13.799 ± 0.021billion years ago. Cosmogony studies the origin of the universe, and cosmography maps the features of the universe.
In Diderot's Encyclopédie,
cosmology is broken down into uranology (the science of the heavens),
aerology (the science of the air), geology (the science of the
continents), and hydrology (the science of waters).
Metaphysical cosmology has also been described as the placing of
humans in the universe in relationship to all other entities. This is
exemplified by Marcus Aurelius's
observation that a man's place in that relationship: "He who does not
know what the world is does not know where he is, and he who does not
know for what purpose the world exists, does not know who he is, nor
what the world is."
Physical cosmology is the branch of physics and astrophysics that
deals with the study of the physical origins and evolution of the
universe. It also includes the study of the nature of the universe on a
large scale. In its earliest form, it was what is now known as "celestial mechanics," the study of the heavens. Greek philosophers Aristarchus of Samos, Aristotle, and Ptolemy proposed different cosmological theories. The geocentricPtolemaic system was the prevailing theory until the 16th century when Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei, proposed a heliocentric system. This is one of the most famous examples of epistemological rupture in physical cosmology.
Isaac Newton's Principia Mathematica, published in 1687, was the first description of the law of universal gravitation. It provided a physical mechanism for Kepler's laws
and also allowed the anomalies in previous systems, caused by
gravitational interaction between the planets, to be resolved. A
fundamental difference between Newton's cosmology and those preceding it
was the Copernican principle—that the bodies on Earth obey the same physical laws as all celestial bodies. This was a crucial philosophical advance in physical cosmology.
Modern scientific cosmology is widely considered to have begun in 1917 with Albert Einstein's publication of his final modification of general relativity in the paper "Cosmological Considerations of the General Theory of Relativity" (although this paper was not widely available outside of Germany until the end of World War I). General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild, and Arthur Eddington to explore its astronomical ramifications, which enhanced the ability of astronomers to study very distant objects. Physicists began changing the assumption that the universe was static and unchanging. In 1922, Alexander Friedmann introduced the idea of an expanding universe that contained moving matter.
In parallel to this dynamic approach to cosmology, one long-standing
debate about the structure of the cosmos was coming to a climax – the Great Debate (1917 to 1922) – with early cosmologists such as Heber Curtis and Ernst Öpik determining that some nebulae seen in telescopes were separate galaxies far distant from our own.
While Heber Curtis argued for the idea that spiral nebulae were star
systems in their own right as island universes, Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Waystar system only. This difference of ideas came to a climax with the organization of the Great Debate on 26 April 1920 at the meeting of the U.S. National Academy of Sciences in Washington, D.C. The debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda Galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way.
Since around 1990, several dramatic advances in observational cosmology
have transformed cosmology from a largely speculative science into a
predictive science with precise agreement between theory and
observation. These advances include observations of the microwave
background from the COBE, WMAP and Planck satellites, large new galaxy redshift surveys including 2dfGRS and SDSS, and observations of distant supernovae and gravitational lensing. These observations matched the predictions of the cosmic inflation theory, a modified Big Bang theory, and the specific version known as the Lambda-CDM model. This has led many to refer to modern times as the "golden age of cosmology".
In 2014, the BICEP2 collaboration claimed that they had detected the imprint of gravitational waves in the cosmic microwave background.
However, this result was later found to be spurious: the supposed
evidence of gravitational waves was in fact due to interstellar dust.
Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation and eschatology.
Creation myths are found in most religions, and are typically split
into five different classifications, based on a system created by Mircea Eliade and his colleague Charles Long.
Types of Creation Myths based on similar motifs:
Creation ex nihilo in which the creation is through the thought, word, dream or bodily secretions of a divine being.
Earth diver creation in which a diver, usually a bird or amphibian sent by a creator, plunges to the seabed through a primordial ocean to bring up sand or mud which develops into a terrestrial world.
Emergence myths in which progenitors pass through a series of worlds and metamorphoses until reaching the present world.
Creation by the dismemberment of a primordial being.
Creation by the splitting or ordering of a primordial unity such as the cracking of a cosmic egg or a bringing order from chaos.
Philosophy
Representation of the observable universe on a logarithmic scale.
Distance from the Sun increases from center to edge. Planets and other
celestial bodies were enlarged to appreciate their shapes.
Cosmology deals with the world as the totality of space, time and all
phenomena. Historically, it has had quite a broad scope, and in many
cases was found in religion.
Some questions about the Universe are beyond the scope of scientific
inquiry but may still be interrogated through appeals to other
philosophical approaches like dialectics. Some questions that are included in extra-scientific endeavors may include: Charles Kahn, an important historian of philosophy, attributed the origins of ancient Greek cosmology to Anaximander.
What is the ultimate reason (if any) for the existence of the universe? Does the cosmos have a purpose? (see teleology)
Does the existence of consciousness have a role in the existence of
reality? How do we know what we know about the totality of the cosmos?
Does cosmological reasoning reveal metaphysical truths? (see epistemology)
Primal matter remains manifest for 311.04 trillion years and unmanifest for an equal length of time. The universe remains manifest for 4.32 billion years and unmanifest
for an equal length of time. Innumerable universes exist
simultaneously. These cycles have and will last forever, driven by
desires.
According to Zoroastrian Cosmology, the universe is the
manifestation of perpetual conflict between Existence and non-existence,
Good and evil and light and darkness. the universe will remain in this
state for 12000 years; at the time of resurrection, the two elements
will be separated again.
Jain cosmology considers the loka,
or universe, as an uncreated entity, existing since infinity, the shape
of the universe as similar to a man standing with legs apart and arm
resting on his waist. This Universe, according to Jainism, is broad at the top, narrow at the middle and once again becomes broad at the bottom.
The Earth and the Heavens
form a unit within infinite "waters of chaos"; the Earth is flat and
circular, and a solid dome (the "firmament") keeps out the outer
"chaos"-ocean.
The Universe is unchanging, uniform, perfect, necessary, timeless,
and neither generated nor perishable. Void is impossible. Plurality and
change are products of epistemic ignorance derived from sense
experience. Temporal and spatial limits are arbitrary and relative to
the Parmenidean whole.
Prakriti (Matter) and Purusha (Consiouness) Relation
Prakriti (Matter) is the source of the world of becoming. It is pure
potentiality that evolves itself successively into twenty four tattvas
or principles. The evolution itself is possible because Prakriti is always in a state of tension among its constituent strands known as gunas (Sattva (lightness or purity), Rajas (passion or activity), and Tamas (inertia or heaviness)). The cause and effect theory of Sankhya is called Satkaarya-vaada (theory of existent causes), and holds that nothing can really be created from or destroyed into nothingness—all evolution is simply the transformation of primal Nature from one form to another.
Geocentric, cylindrical Earth, infinite in extent, finite time; first purely mechanical model
The Earth floats very still in the centre of the infinite, not supported by anything.
At the origin, after the separation of hot and cold, a ball of flame
appeared that surrounded Earth like bark on a tree. This ball broke
apart to form the rest of the Universe. It resembled a system of hollow
concentric wheels, filled with fire, with the rims pierced by holes like
those of a flute; no heavenly bodies as such, only light through the
holes. Three wheels, in order outwards from Earth: stars (including planets), Moon and a large Sun.
The universe contains only two things: an infinite number of tiny seeds (atoms)
and the void of infinite extent. All atoms are made of the same
substance, but differ in size and shape. Objects are formed from atom
aggregations and decay back into atoms. Incorporates Leucippus' principle of causality: "nothing happens at random; everything happens out of reason and necessity". The universe was not ruled by gods.
Existence of a "Central Fire" at the center of the Universe.
At the center of the Universe is a central fire, around which the Earth, Sun, Moon and planets
revolve uniformly. The Sun revolves around the central fire once a
year, the stars are immobile. The Earth in its motion maintains the same
hidden face towards the central fire, hence it is never seen. First
known non-geocentric model of the Universe.
The Universe is a system made up of heaven and Earth and the elements which are contained in them.
There are "five elements, situated in spheres in five regions, the
less being in each case surrounded by the greater – namely, earth
surrounded by water, water by air, air by fire, and fire by ether – make
up the whole Universe."
The cosmos
is finite and surrounded by an infinite void. It is in a state of flux,
and pulsates in size and undergoes periodic upheavals and
conflagrations.
Static Earth at center, surrounded by heavenly bodies which move in perfect circles, arranged by the will of the Demiurge in order: Moon, Sun, planets and fixed stars. Complex motions repeat every 'perfect' year.
The heavenly bodies move as if attached to a number of Earth-centered concentrical, invisible spheres, each of them rotating around its own and different axis and at different paces.
There are twenty-seven homocentric spheres with each sphere explaining a
type of observable motion for each celestial object. Eudoxus emphasised
that this is a purely mathematical construct of the model in the sense
that the spheres of each celestial body do not exist, it just shows the
possible positions of the bodies.
Geocentric (based on Eudoxus' model), static, steady state, finite extent, infinite time
Static and spherical Earth is surrounded by 43 to 55 concentriccelestial spheres, which are material and crystalline. Universe exists unchanged throughout eternity. Contains a fifth element, called aether, that was added to the four classical elements.
Universe orbits around a stationary Earth. Planets move in circular epicycles,
each having a center that moved in a larger circular orbit (called an
eccentric or a deferent) around a center-point near Earth. The use of equants
added another level of complexity and allowed astronomers to predict
the positions of the planets. The most successful universe model of all
time, using the criterion of longevity. The Almagest (the Great System).
The Earth is at rest in the center of the universe and circled by the Moon, the Sun, three planets and the stars, while Mercury and Venus circle the Sun.
The Earth rotates and the planets move in elliptical orbits
around either the Earth or Sun; uncertain whether the model is
geocentric or heliocentric due to planetary orbits given with respect to
both the Earth and Sun.
Innumerable universes is comparable to the multiverse theory, except nonparallel where each universe is different and individual jiva-atmas
(embodied souls) exist in exactly one universe at a time. All universes
manifest from the same matter, and so they all follow parallel time
cycles, manifesting and unmanifesting at the same time.
Heliocentric with circular planetary orbits, finite extent
First described in De revolutionibus orbium coelestium.
The Sun is in the center of the universe, planets including Earth orbit
the Sun, but the Moon orbits the Earth. The universe is limited by the
sphere of the fixed stars.
Infinite extent, infinite time, homogeneous, isotropic, non-hierarchical
Rejects the idea of a hierarchical universe. Earth and Sun have no
special properties in comparison with the other heavenly bodies. The
void between the stars is filled with aether, and matter is composed of the same four elements (water, earth, fire, and air), and is atomistic, animistic and intelligent.
Kepler's discoveries, marrying mathematics and physics, provided the foundation for the present conception of the Solar System, but distant stars were still seen as objects in a thin, fixed celestial sphere.
Every particle in the universe attracts every other particle. Matter
on the large scale is uniformly distributed. Gravitationally balanced
but unstable.
System of huge swirling whirlpools of aethereal or fine matter
produces gravitational effects. But his vacuum was not empty; all space
was filled with matter.
"Matter without motion". Contains uniformly distributed matter. Uniformly curved spherical space; based on Riemann's hypersphere. Curvature is set equal to Λ. In effect Λ is equivalent to a repulsive force which counteracts gravity. Unstable.
Demands a large variation in G, which decreases with time. Gravity weakens as universe evolves.
Friedmann zero-curvature
Einstein and De Sitter 1932
Expanding flat space
k = 0 ; Λ = 0 Critical density
Curvature constant k = 0. Said to be infinite (but
ambiguous). "Unbounded cosmos of limited extent". Expands forever.
"Simplest" of all known universes. Named after but not considered by
Friedmann. Has a deceleration termq = 1/2, which means that its expansion rate slows down.
Λ is positive and has a magnitude greater than gravity. Universe has
initial high-density state ("primeval atom"). Followed by a two-stage
expansion. Λ is used to destabilize the universe. (Lemaître is
considered the father of the Big Bang model.)
Time is endless and beginningless; thus avoids the beginning-of-time
paradox. Perpetual cycles of Big Bang followed by Big Crunch.
(Einstein's first choice after he rejected his 1917 model.)
Static Einstein 1917 universe with its instability disturbed into
expansion mode; with relentless matter dilution becomes a De Sitter
universe. Λ dominates gravity.
Rejects general relativity and the expanding space paradigm. Gravity
not included as initial assumption. Obeys cosmological principle and special relativity;
consists of a finite spherical cloud of particles (or galaxies) that
expands within an infinite and otherwise empty flat space. It has a
center and a cosmic edge (surface of the particle cloud) that expands at
light speed. Explanation of gravity was elaborate and unconvincing.
Class of universes that are homogeneous and isotropic. Spacetime
separates into uniformly curved space and cosmic time common to all
co-moving observers. The formulation system is now known as the FLRW or
Robertson–Walker metrics of cosmic time and curved space.
Matter creation rate maintains constant density. But since matter
creation rate must be exactly balanced with the space expansion rate the
system is unstable.
Cellular universe, expanding by means of matter–antimatter annihilation
Based on the concept of plasma cosmology. The universe is viewed as "meta-galaxies" divided by double layers and thus a bubble-like nature. Other universes are formed from other bubbles. Ongoing cosmic matter-antimatterannihilations keep the bubbles separated and moving apart preventing them from interacting.
Based on the concept of hot inflation. The universe is viewed as a
multiple quantum flux – hence its bubble-like nature. Other universes
are formed from other bubbles. Ongoing cosmic expansion kept the bubbles
separated and moving apart.
Multiverse
based on the concept of cold inflation, in which inflationary events
occur at random each with independent initial conditions; some expand
into bubble universes supposedly like the entire cosmos. Bubbles
nucleate in a spacetime foam.
Phantom dark energy
fragments universe into large number of disconnected patches. The
observable patch contracts containing only dark energy with zero entropy.
Table notes: the term "static" simply means not expanding and not contracting. Symbol G represents Newton's gravitational constant; Λ (Lambda) is the cosmological constant.
