An interstellar cloud is generally an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in the space between the star systems in a galaxy. Depending on the density, size, and temperature of a given cloud, its hydrogen can be neutral, making an H I region; ionized, or plasma making it an H II region; or molecular, which are referred to simply as molecular clouds, or sometime dense clouds. Neutral and ionized clouds are sometimes also called diffuse clouds. An interstellar cloud is formed by the gas and dust particles from a red giant in its later life.
Chemical compositions
The chemical composition of interstellar clouds is determined by studying electromagnetic radiation that they emanate, and we receive – from radio waves through visible light, to gamma rays on the electromagnetic spectrum – that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of electromagnetic radiation, which are characteristic of certain molecules' spectra. Some interstellar clouds are cold and tend to give out electromagnetic radiation of large wavelengths.
A map of the abundance of these molecules can be made, enabling an
understanding of the varying composition of the clouds. In hot clouds,
there are often ions of many elements, whose spectra can be seen in visible and ultraviolet light.
Radio telescopes can also scan over the frequencies from one
point in the map, recording the intensities of each type of molecule.
Peaks of frequencies mean that an abundance of that molecule or atom is
present in the cloud. The height of the peak is proportional to the
relative percentage that it makes up.
Unexpected chemicals detected in interstellar clouds
Until recently, the rates of reactions in interstellar clouds were
expected to be very slow, with minimal products being produced due to
the low temperature and density of the clouds. However, organic molecules were observed in the spectra that scientists would not have expected to find under these conditions, such as formaldehyde, methanol, and vinyl alcohol.
The reactions needed to create such substances are familiar to
scientists only at the much higher temperatures and pressures of earth
and earth-based laboratories. The fact that they were found indicates
that these chemical reactions
in interstellar clouds take place faster than suspected, likely in
gas-phase reactions unfamiliar to organic chemistry as observed on
earth. These reactions are studied in the CRESU experiment.
Interstellar clouds also provide a medium to study the presence
and proportions of metals in space. The presence and ratios of these
elements may help develop theories on the means of their production,
especially when their proportions are inconsistent with those expected
to arise from stars as a result of fusion and thereby suggest alternate means, such as cosmic ray spallation.
These interstellar clouds possess a velocity higher than can be explained by the rotation of the Milky Way. By definition, these clouds must have a vlsr greater than 90 km s−1, where vlsr is the local standard rest velocity. They are detected primarily in the 21 cm line of neutral hydrogen, and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.
Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, or tidally-displacedmatter drawn away from other galaxies or members of the Local Group. An example of the latter is the Magellanic Stream. To narrow down the origin of these clouds, a better understanding of their distances and metallicity is needed.
High-velocity clouds are identified with an HVC prefix, as with HVC 127-41-330.
The radio source consists of three components: the supernova remnantSagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*
(read "A-star"). These three overlap: Sagittarius A East is the
largest, West appears off-center within East, and A* is at the center of
West.
Sagittarius spiral arm
A
study was done with the measured parallaxes and motions of 10 massive
regions in the Sagittarius spiral arm of the Milky Way where stars are
formed. Data was gathered using the BeSSeL Survey with the VLBA,
and the results were synthesized to discover the physical properties of
these sections (called the Galactocentric azimuth, around −2 and 65
degrees). The results were that the spiral pitch angle of the arms is
7.3 ±
1.5 degrees, and the half-width of the arms of the Milky Way were found
to be 0.2 kpc. The nearest arm from the Sun is around 1.4 ± 0.2 kpc
away.
Sagittarius A East
This feature is approximately 25 light-years
in width and has the attributes of a supernova remnant from an
explosive event that occurred between 35,000 and 100,000 BC. However, it
would take 50 to 100 times more energy than a standard supernova explosion
to create a structure of this size and energy. It is conjectured that
Sgr A East is the remnant of the explosion of a star that was
gravitationally compressed as it made a close approach to the central black hole.
Sagittarius A West
Surface brightness and velocity field of the inner part of Sagittarius A West
Sgr A West has the appearance of a three-arm spiral, from the point
of view of the Earth. For this reason, it is also known as the
"Minispiral". This appearance and nickname are misleading, though: the
three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds,
which orbit and fall onto Sagittarius A* at velocities as high as 1,000
kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB stars have been identified so far) that also occupy the central parsec.
Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics
of the Northern Arm cloud of Sgr A West suggest that it once was a
clump in the CND, which fell due to some perturbation, perhaps the
supernova explosion responsible for Sgr A East. The Northern Arm appears
as a very bright North—South ridge of emission, but it extends far to
the East and can be detected as a dim extended source.
The Western Arc (outside the field of view of the image shown in
the right) is interpreted as the ionized inner surface of the CND. The
Eastern Arm and the Bar seem to be two additional large clouds similar
to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.
On top of these large scale structures (of the order of a few
light-years in size), many smaller cloudlets and holes inside the large
clouds can be seen. The most prominent of these perturbations is the
Minicavity, which is interpreted as a bubble blown inside the Northern
Arm by the stellar wind of a massive star, which is not clearly identified.
Astronomers now have evidence that there is a supermassive black hole at the center of the galaxy.
Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible
candidate for the location of this supermassive black hole. The Very Large Telescope at Chile and Keck Telescope
at Hawaii have detected stars orbiting Sgr A* at speeds greater than
that of any other stars in the galaxy. One star, designated S2, was calculated to orbit Sgr A* at speeds of over 5,000 kilometers per second at its closest approach.
A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the event horizon,
as theorists predicted would happen. Rather, it disintegrated,
suggesting that G2 and a previous gas cloud, G1, were star remnants with
larger gravitational fields than gas clouds.
In September 2019, scientists found that Sagittarius A* had been
consuming nearby matter at a much faster rate than usual over the
previous year. Researchers speculated that this could mean that the
black hole is entering a new phase, or that Sagittarius A* had stripped
the outer layer of G2 when it passed through.
In the television show Community, Pierce Hawthorne mentions that in his opinion, Sagittarius A is the only black hole worth studying.
In the H.P. Lovecraft Cthulhu Mythos, Sagittarius A is said to be the location of where the Nuclear Chaos, Azathoth, dwells.
In the final arc of the Sailor Moon
manga series, "Sagittarius Zero Star" is the location of the Galaxy
Cauldron, a fictional artifact that serves as the birthplace of all life
in the Milky Way.
The Galactic Center, as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image.Marked location of the Galactic Center.A starchart of the night sky towards the Galactic Center.
There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf–Rayet stars from star formation in the region around 1 million years ago. The core stars are a small part within the much wider galactic bulge.
Discovery
Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft (low-energy) X-raywavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard (high-energy) X-ray, infrared, submillimetre, and radio wavelengths.
Immanuel Kant stated in Universal Natural History and Theory of the Heavens (1755) that a large star was at the center of the Milky Way Galaxy, and that Sirius might be the star. Harlow Shapley stated in 1918 that the halo of globular clusters surrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the dark molecular clouds in the area blocked the view for optical astronomy. In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime blackout conditions in nearby Los Angeles to conduct a search for the center with the 100-inch (250 cm) Hooker Telescope. He found that near the star Alnasl
(Gamma Sagittarii) there is a one-degree-wide void in the interstellar
dust lanes, which provides a relatively clear view of the swarms of
stars around the nucleus of the Milky Way Galaxy. This gap has been known as Baade's Window ever since.
At Dover Heights in Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at the CSIRO, led by Joseph Lade Pawsey, used "sea interferometry" to discover some of the first interstellar and intergalactic radio sources, including Taurus A, Virgo A and Centaurus A.
By 1954 they had built an 80-foot (24 m) fixed dish antenna and used it
to make a detailed study of an extended, extremely powerful belt of
radio emission that was detected in Sagittarius. They named an intense
point-source near the center of this belt Sagittarius A,
and realised that it was located at the very center of the Galaxy,
despite being some 32 degrees south-west of the conjectured galactic
center of the time.
In July 2022, astronomers reported the discovery of massive amounts of prebiotic molecules, including some associated with RNA, in the Galactic Center of the Milky Way Galaxy.
Distance to the Galactic Center
The exact distance between the Solar System and the Galactic Center is not certain, although estimates since 2000 have remained within the range 24–28.4 kilolight-years (7.4–8.7 kiloparsecs). The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:
7.4±0.2(stat) ± 0.2(syst) or 7.4±0.3 kpc (≈24±1 kly)
7.62±0.32 kpc (≈24.8±1 kly)
7.7±0.7 kpc (≈25.1±2.3 kly)
7.94 or 8.0±0.5 kpc (≈26±1.6 kly)
7.98±0.15(stat) ± 0.20(syst) or 8.0±0.25 kpc (≈26±0.8 kly)
An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by numerous effects, which include: an ambiguous reddening law;
a bias for smaller values of the distance to the Galactic Center
because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction; and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.
The nature of the Milky Way's bar,
which extends across the Galactic Center, is also actively debated,
with estimates for its half-length and orientation spanning between 1–5
kpc (short or a long bar) and 10–50°. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. The bar is delineated by red-clump stars (see also red giant); however, RR Lyrae variables do not trace a prominent Galactic bar. The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.
A study in 2008 which linked radio telescopes in Hawaii, Arizona and California (Very-long-baseline interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3 AU) For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers (1.0 AU), whereas the distance of Mercury from the Sun at closest approach (perihelion)
is 46 million kilometers (0.3 AU). Thus, the diameter of the radio
source is slightly less than the distance from Mercury to the Sun.
Scientists at the Max Planck Institute for Extraterrestrial Physics
in Germany using Chilean telescopes have confirmed the existence of a
supermassive black hole at the Galactic Center, on the order of 4.3
million solar masses. Later studies have estimated a mass of 3.7 millionor 4.1 million solar masses.
On 5 January 2015, NASA reported observing an X-ray
flare 400 times brighter than usual, a record-breaker, from Sagittarius
A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.
There
is a supermassive black hole in the bright white area to the right of
the center of the image. This composite photograph covers about half of a
degree.
Gamma- and X-ray emitting Fermi bubbles
Galactic gamma- and X-ray bubbles
In November 2010, it was announced that two large elliptical lobe structures of energetic plasma, termed bubbles, which emit gamma- and X-rays, were detected astride the Milky Way galaxy's core. Termed Fermi or eRosita bubbles, they extend up to about 25,000 light years above and below the Galactic Center.
The galaxy's diffuse gamma-ray fog hampered prior observations, but the
discovery team led by D. Finkbeiner, building on research by G. Dobler,
worked around this problem. The 2014 Bruno Rossi Prize went to Tracy Slatyer, Douglas Finkbeiner, and Meng Su "for their discovery, in gamma rays, of the large unanticipated Galactic structure called the Fermi bubbles".
The origin of the bubbles is being researched.
The bubbles are connected and seemingly coupled, via energy transport,
to the galactic core by columnar structures of energetic plasma termed chimneys. In 2020, for the first time, the lobes were seen in visible light and optical measurements were made. By 2022, detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole.
The central cubic parsec around Sagittarius A* contains around 10 million stars. Although most of them are old red giant stars, the Galactic Center is also rich in massive stars. More than 100 OB and Wolf–Rayet stars have been identified there so far. They seem to have all been formed in a single star formation event a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected the tidal forces from the central black hole to prevent their formation. This paradox of youth is even stronger for stars that are on very tight orbits around Sagittarius A*, such as S2 and S0-102. The scenarios invoked to explain this formation involve either star formation in a massive star cluster
offset from the Galactic Center that would have migrated to its current
location once formed, or star formation within a massive, compact gas accretion disk
around the central black-hole. Current evidence favors the latter
theory, as formation through a large accretion disk is more likely to
lead to the observed discrete edge of the young stellar cluster at
roughly 0.5 parsec.
Most of these 100 young, massive stars seem to be concentrated within
one or two disks, rather than randomly distributed within the central
parsec. This observation however does not allow definite conclusions to be drawn at this point.
Star formation does not seem to be occurring currently at the
Galactic Center, although the Circumnuclear Disk of molecular gas that
orbits the Galactic Center at two parsecs seems a fairly favorable site
for star formation. Work presented in 2002 by Antony Stark and Chris
Martin mapping the gas density in a 400-light-year region around the Galactic Center has revealed an accumulating ring with a mass several million times that of the Sun and near the critical density for star formation. They predict that in approximately 200 million years there will be an episode of starburst
in the Galactic Center, with many stars forming rapidly and undergoing
supernovae at a hundred times the current rate. This starburst may also
be accompanied by the formation of galactic relativistic jets as matter falls into the central black hole. It is thought that the Milky Way undergoes a starburst of this sort every 500 million years.
In addition to the paradox of youth, there is also a "conundrum
of old age" associated with the distribution of the old stars at the
Galactic Center. Theoretical models had predicted that the old
stars—which far outnumber young stars—should have a steeply-rising
density near the black hole, a so-called Bahcall–Wolf cusp.
Instead, it was discovered in 2009 that the density of the old stars
peaks at a distance of roughly 0.5 parsec from Sgr A*, then falls
inward: instead of a dense cluster, there is a "hole", or core, around the black hole. Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.
For instance, although the black hole would eat stars near it, creating
a region of low density, this region would be much smaller than a
parsec. Because the observed stars are a fraction of the total number,
it is theoretically possible that the overall stellar distribution is
different from what is observed, although no plausible models of this
sort have been proposed yet.
A quasi-star would have resulted from the core of a large protostar collapsing into a black hole,
where the outer layers of the protostar are massive enough to absorb
the resulting burst of energy without being blown away or falling into
the black hole, as occurs with modern supernovae. The star would have been almost 10 million solar masses at this point. Quasi-stars may have also formed from dark matter halos
(up to 100 million solar masses; for reference, some small galaxies
have only 5 million solar masses) drawing in enormous amounts of gas via
gravity, which can produce supermassive stars with tens of thousands of solar masses.
Formation of quasi-stars could only happen early in the development of
the Universe, before hydrogen and helium were contaminated by heavier
elements; thus, they may have been very massive Population III stars. Such stars would dwarf VY Canis Majoris and Stephenson 2 DFK 1, both among the largest known modern stars, in size.
Once the black hole had formed at the core of the protostar, it would continue generating a large amount of radiant energy from the infall of stellar material. This constant outburst of energy would counteract the force of gravity, creating an equilibrium similar to the one that supports modern fusion-based stars. Quasi-stars would have had a short maximum lifespan, approximately 7 million years, during which the core black hole would have grown to about 1,000–10,000 solar masses (2×1033–2×1034 kg).These intermediate-mass black holes have been suggested as the progenitors of modern supermassive black holes such as the one in the center of the Galaxy.
Quasi-stars are predicted to have had surface temperatures higher than 10,000 K (9,700 °C). At these temperatures, and with a radius of approximately 800 thousand times that of the Sun (as large as the Solar System), each one would be about as luminous as a small galaxy.
As a quasi-star cools over time, its outer envelope would become
transparent, until further cooling to a limiting temperature of 4,000 K
(3,730 °C) and growing to a size of 30 solar systems in terms of length. This limiting temperature would mark the end of the quasi-star's life since there is no hydrostatic equilibrium at or below this limiting temperature. The object would then quickly dissipate, leaving behind the intermediate mass black hole.
In astronomy, the lithium problem or lithium discrepancy refers to the discrepancy between the primordial abundance of lithium as inferred from observations of metal-poor (Population II) halo stars in our galaxy and the amount that should theoretically exist due to Big Bang nucleosynthesis+WMAP cosmic baryon density predictions of the CMB. Namely, the most widely accepted models of the Big Bang suggest that three times as much primordial lithium, in particular lithium-7, should exist. This contrasts with the observed abundance of isotopes of hydrogen (1H and 2H) and helium (3He and 4He) that are consistent with predictions. The discrepancy is highlighted in a so-called "Schramm plot", named in honor of astrophysicist David Schramm, which depicts these primordial abundances as a function of cosmic baryon content from standard BBN predictions.
This "Schramm plot" depicts primordial abundances of 4He, D, 3He, and 7Li as a function of cosmic baryon content from standard BBN predictions. CMB predictions of 7Li (narrow vertical bands, at 95% CL) and the BBN D + 4He
concordance range (wider vertical bands, at 95% CL) should overlap with
the observed light element abundances (yellow boxes) to be in
agreement. This occurs in 4He and is well constrained in D, but is not the case for 7Li, where the observed Li observations lie a factor of 3−4 below the BBN+WMAP prediction.
Origin of lithium
Minutes
after the Big Bang, the universe was made almost entirely of hydrogen
and helium, with trace amounts of lithium and beryllium, and negligibly
small abundances of all heavier elements.
Lithium synthesis in the Big Bang
Big
Bang nucleosynthesis produced both lithium-7 and beryllium-7, and
indeed the latter dominates the primordial synthesis of mass 7 nuclides.
On the other hand, the Big Bang produced lithium-6 at levels more than
1000 times smaller.
7 4Be later decayed via electron capture (half-life 53.22 days) into 7 3Li ,
so that the observable primordial lithium abundance essentially sums primordial 7 3Li and radiogenic lithium from the decay of 7 4Be .
The amount of lithium generated in the Big Bang can be calculated. Hydrogen-1 is the most abundant nuclide, comprising roughly 92% of the atoms in the Universe, with helium-4 second at 8%. Other isotopes including 2H, 3H, 3He, 6Li, 7Li, and 7Be are much rarer; the estimated abundance of primordial lithium is 10−10 relative to hydrogen. The calculated abundance and ratio of 1H and 4He is in agreement with data from observations of young stars.
The P-P II branch is dominant at temperatures of 14 to 23 MK.
Stable nuclides of the first few elements
Observed abundance of lithium
Despite
the low theoretical abundance of lithium, the actual observable amount
is less than the calculated amount by a factor of 3–4. This contrasts with the observed abundance of isotopes of hydrogen (1H and 2H) and helium (3He and 4He) that are consistent with predictions.
Abundances
of the chemical elements in the Solar System. Hydrogen and helium are
most common, residuals within the paradigm of the Big Bang.
Li, Be and B are rare because they are poorly synthesized in the Big
Bang and also in stars; the main source of these elements is cosmic ray spallation.
Older stars seem to have less lithium than they should, and some younger stars have much more.
One proposed model is that lithium produced during a star's youth sinks
beneath the star's atmosphere (where it is obscured from direct
observation) due to effects the authors describe as "turbulent mixing"
and "diffusion," which are suggested to increase or accumulate as the
star ages. Spectroscopic observations of stars in NGC 6397,
a metal-poor globular cluster, are consistent with an inverse relation
between lithium abundance and age, but a theoretical mechanism for
diffusion has not been formalized. Though it transmutes into two atoms of helium due to collision with a proton
at temperatures above 2.4 million degrees Celsius (most stars easily
attain this temperature in their interiors), lithium is more abundant
than current computations would predict in later-generation stars.
Nova Centauri 2013 is the first in which evidence of lithium has been found.
Lithium is also found in brown dwarf
substellar objects and certain anomalous orange stars. Because lithium
is present in cooler, less massive brown dwarfs, but is destroyed in
hotter red dwarf
stars, its presence in the stars' spectra can be used in the "lithium
test" to differentiate the two, as both are smaller than the Sun.
Less lithium in Sun-like stars with planets
Sun-like stars without planets have 10 times the lithium as Sun-like stars with planets in a sample of 500 stars. The Sun's surface layers have less than 1% the lithium of the original formation protosolar gas clouds despite the surface convective zone not being quite hot enough to burn lithium.
It is suspected that the gravitational pull of planets might enhance
the churning up of the star's surface, driving the lithium to hotter
cores where lithium burning occurs. The absence of lithium could also be a way to find new planetary systems.
However, this claimed relationship has become a point of contention in
the planetary astrophysics community, being frequently denied but also supported.
Higher than expected lithium in metal-poor stars
Certain orange stars can also contain a high concentration of lithium.
Those orange stars found to have a higher than usual concentration of
lithium orbit massive objects—neutron stars or black holes—whose gravity
evidently pulls heavier lithium to the surface of a hydrogen-helium
star, causing more lithium to be observed.
Proposed solutions
Possible solutions fall into three broad classes.
Astrophysical solutions
Considering
the possibility that BBN predictions are sound, the measured value of
the primordial lithium abundance should be in error and astrophysical
solutions offer revision to it. For example, systematic errors,
including ionization correction and inaccurate stellar temperatures
determination could affect Li/H ratios in stars. Furthermore, more
observations on lithium depletion remain important since present lithium
levels might not reflect the initial abundance in the star. In summary,
accurate measurements of the primordial lithium abundance is the
current focus of progress, and it could be possible that the final
answer does not lie in astrophysical solutions.
Some astronomers suggest that the velocities of nucleons do not
follow Maxwell-Boltzmann distribution. They test the framework of
Tsallis non-extensive statistics.Their result suggest that
1.069<q<1.082 is a possible new solution to the cosmological
lithium problem.
Nuclear physics solutions
When one considers the possibility that the measured primordial lithium abundance is correct and based on the Standard Model
of particle physics and the standard cosmology, the lithium problem
implies errors in the BBN light element predictions. Although standard
BBN rests on well-determined physics, the weak and strong interactions
are complicated for BBN and therefore might be the weak point in
standard BBN calculation.
Firstly, incorrect or missing reactions could give rise to the
lithium problem. For incorrect reactions, major thoughts lie within
revision to cross section errors and standard thermonuclear rates according to recent studies.
Second, starting from Fred Hoyle's discovery of a resonance in carbon-12, an important factor in the triple-alpha process,
resonance reactions, some of which might have evaded experimental
detection or whose effects have been underestimated, become possible
solutions to the lithium problem.
BBC Science Focus
wrote in 2023 that "recent research seems to completely discount" such
theories; the magazine held that mainstream lithium nucleosynthesis
calculations are probably correct.
Solutions beyond the Standard Model
Under the assumptions of all correct calculation, solutions beyond the existing Standard Model or standard cosmology might be needed.
Dark matter decay and supersymmetry
provide one possibility, in which decaying dark matter scenarios
introduce a rich array of novel processes that can alter light elements
during and after BBN, and find the well-motivated origin in
supersymmetric cosmologies. With the fully operational Large Hadron Collider (LHC), much of minimal supersymmetry lies within reach, which would revolutionize particle physics and cosmology if discovered; however, results from the ATLAS experiment in 2020 have excluded many supersymmetric models.
Changing fundamental constants can be one possible solution, and it implies that first, atomic transitions in metals residing in high-redshift
regions might behave differently from our own. Additionally, Standard
Model couplings and particle masses might vary; third, variation in
nuclear physics parameters is needed.
Nonstandard cosmologies indicate variation of the baryon to
photon ratio in different regions. One proposal is a result of
large-scale inhomogeneities in cosmic density, different from
homogeneity defined in the cosmological principle. However, this possibility requires a large amount of observations to test it.
