Weak gravitational lensing

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Weak_gravitational_lensing

While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime, in which the deflection is impossible to detect in a single background source. However, even in these cases, the presence of the foreground mass can be detected, by way of a systematic alignment of background sources around the lensing mass. Weak gravitational lensing is thus an intrinsically statistical measurement, but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamical state.

Methodology

Distortions of the type produced by lensing, acting on circles and a distribution of ellipses similar to that of real galaxies. The distortion shown here is greatly exaggerated relative to real astronomical systems.

Gravitational lensing acts as a coordinate transformation that distorts the images of background objects (usually galaxies) near a foreground mass. The transformation can be split into two terms, the convergence and shear. The convergence term magnifies the background objects by increasing their size, and the shear term stretches them tangentially around the foreground mass.

To measure this tangential alignment, it is necessary to measure the ellipticities of the background galaxies and construct a statistical estimate of their systematic alignment. The fundamental problem is that galaxies are not intrinsically circular, so their measured ellipticity is a combination of their intrinsic ellipticity and the gravitational lensing shear. Typically, the intrinsic ellipticity is much greater than the shear (by a factor of 3-300, depending on the foreground mass). The measurements of many background galaxies must be combined to average down this "shape noise". The orientation of intrinsic ellipticities of galaxies should be almost entirely random, so any systematic alignment between multiple galaxies can generally be assumed to be caused by lensing.

Another major challenge for weak lensing is correction for the point spread function (PSF) due to instrumental and atmospheric effects, which causes the observed images to be smeared relative to the "true sky". This smearing tends to make small objects more round, destroying some of the information about their true ellipticity. As a further complication, the PSF typically adds a small level of ellipticity to objects in the image, which is not at all random, and can in fact mimic a true lensing signal. Even for the most modern telescopes, this effect is usually at least the same order of magnitude as the gravitational lensing shear, and is often much larger. Correcting for the PSF requires building for the telescope a model for how it varies across the field. Stars in our own galaxy provide a direct measurement of the PSF, and these can be used to construct such a model, usually by interpolating between the points where stars appear on the image. This model can then be used to reconstruct the "true" ellipticities from the smeared ones. Ground-based and space-based data typically undergo distinct reduction procedures due to the differences in instruments and observing conditions.

Angular diameter distances to the lenses and background sources are important for converting the lensing observables to physically meaningful quantities. These distances are often estimated using photometric redshifts when spectroscopic redshifts are unavailable. Redshift information is also important in separating the background source population from other galaxies in the foreground, or those associated with the mass responsible for the lensing. With no redshift information, the foreground and background populations can be split by an apparent magnitude or a color cut, but this is much less accurate.

Weak lensing by clusters of galaxies

The effects of foreground galaxy cluster mass on background galaxy shapes. The upper left panel shows (projected onto the plane of the sky) the shapes of cluster members (in yellow) and background galaxies (in white), ignoring the effects of weak lensing. The lower right panel shows this same scenario, but includes the effects of lensing. The middle panel shows a 3-d representation of the positions of cluster and source galaxies, relative to the observer. Note that the background galaxies appear stretched tangentially around the cluster.

Galaxy clusters are the largest gravitationally bound structures in the Universe with approximately 80% of cluster content in the form of dark matter. The gravitational fields of these clusters deflect light-rays traveling near them. As seen from Earth, this effect can cause dramatic distortions of a background source object detectable by eye such as multiple images, arcs, and rings (cluster strong lensing). More generally, the effect causes small, but statistically coherent, distortions of background sources on the order of 10% (cluster weak lensing). Abell 1689, CL0024+17, and the Bullet Cluster are among the most prominent examples of lensing clusters.

History

The effects of cluster strong lensing were first detected by Roger Lynds of the National Optical Astronomy Observatories and Vahe Petrosian of Stanford University who discovered giant luminous arcs in a survey of galaxy clusters in the late 1970s. Lynds and Petrosian published their findings in 1986 without knowing the origin of the arcs. In 1987, Genevieve Soucail of the Toulouse Observatory and her collaborators presented data of a blue ring-like structure in Abell 370 and proposed a gravitational lensing interpretation. The first cluster weak lensing analysis was conducted in 1990 by J. Anthony Tyson of Bell Laboratories and collaborators. Tyson et al. detected a coherent alignment of the ellipticities of the faint blue galaxies behind both Abell 1689 and CL 1409+524. Lensing has been used as a tool to investigate a tiny fraction of the thousands of known galaxy clusters.

Historically, lensing analyses were conducted on galaxy clusters detected via their baryon content (e.g. from optical or X-ray surveys). The sample of galaxy clusters studied with lensing was thus subject to various selection effects; for example, only the most luminous clusters were investigated. In 2006, David Wittman of the University of California at Davis and collaborators published the first sample of galaxy clusters detected via their lensing signals, completely independent of their baryon content. Clusters discovered through lensing are subject to mass selection effects because the more massive clusters produce lensing signals with higher signal-to-noise.

Observational products

The projected mass density can be recovered from the measurement of the ellipticities of the lensed background galaxies through techniques that can be classified into two types: direct reconstruction and inversion. However, a mass distribution reconstructed without knowledge of the magnification suffers from a limitation known as the mass sheet degeneracy, where the cluster surface mass density κ can be determined only up to a transformation ${\displaystyle \kappa \rightarrow \kappa ^{\prime }=\lambda \kappa +(1-\lambda )}$ where λ is an arbitrary constant. This degeneracy can be broken if an independent measurement of the magnification is available because the magnification is not invariant under the aforementioned degeneracy transformation.

Given a centroid for the cluster, which can be determined by using a reconstructed mass distribution or optical or X-ray data, a model can be fit to the shear profile as a function of clustrocentric radius. For example, the singular isothermal sphere (SIS) profile and the Navarro-Frenk-White (NFW) profile are two commonly used parametric models. Knowledge of the lensing cluster redshift and the redshift distribution of the background galaxies is also necessary for estimation of the mass and size from a model fit; these redshifts can be measured precisely using spectroscopy or estimated using photometry. Individual mass estimates from weak lensing can only be derived for the most massive clusters, and the accuracy of these mass estimates are limited by projections along the line of sight.

Scientific implications

Image of the Bullet Cluster from the Hubble Space Telescope with total mass contours (dominated by dark matter) from a lensing analysis overlaid.

Cluster mass estimates determined by lensing are valuable because the method requires no assumption about the dynamical state or star formation history of the cluster in question. Lensing mass maps can also potentially reveal "dark clusters," clusters containing overdense concentrations of dark matter but relatively insignificant amounts of baryonic matter. Comparison of the dark matter distribution mapped using lensing with the distribution of the baryons using optical and X-ray data reveals the interplay of the dark matter with the stellar and gas components. A notable example of such a joint analysis is the so-called Bullet Cluster. The Bullet Cluster data provide constraints on models relating light, gas, and dark matter distributions such as Modified Newtonian dynamics (MOND) and Λ-Cold Dark Matter (Λ-CDM).

In principle, since the number density of clusters as a function of mass and redshift is sensitive to the underlying cosmology, cluster counts derived from large weak lensing surveys should be able to constrain cosmological parameters. In practice, however, projections along the line of sight cause many false positives. Weak lensing can also be used to calibrate the mass-observable relation via a stacked weak lensing signal around an ensemble of clusters, although this relation is expected to have an intrinsic scatter. In order for lensing clusters to be a precision probe of cosmology in the future, the projection effects and the scatter in the lensing mass-observable relation need to be thoroughly characterized and modeled.

Galaxy-galaxy lensing

Galaxy-galaxy lensing is a specific type of weak (and occasionally strong) gravitational lensing, in which the foreground object responsible for distorting the shapes of background galaxies is itself an individual field galaxy (as opposed to a galaxy cluster or the large-scale structure of the cosmos). Of the three typical mass regimes in weak lensing, galaxy-galaxy lensing produces a "mid-range" signal (shear correlations of ~1%) that is weaker than the signal due to cluster lensing, but stronger than the signal due to cosmic shear.

History

J.A. Tyson and collaborators first postulated the concept of galaxy-galaxy lensing in 1984, though the observational results of their study were inconclusive. It was not until 1996 that evidence of such distortion was tentatively discovered, with the first statistically significant results not published until the year 2000. Since those initial discoveries, the construction of larger, high resolution telescopes and the advent of dedicated wide field galaxy surveys have greatly increased the observed number density of both background source and foreground lens galaxies, allowing for a much more robust statistical sample of galaxies, making the lensing signal much easier to detect. Today, measuring the shear signal due to galaxy-galaxy lensing is a widely used technique in observational astronomy and cosmology, often used in parallel with other measurements in determining physical characteristics of foreground galaxies.

Stacking

Much like in cluster-scale weak lensing, detection of a galaxy-galaxy shear signal requires one to measure the shapes of background source galaxies, and then look for statistical shape correlations (specifically, source galaxy shapes should be aligned tangentially, relative to the lens center.) In principle, this signal could be measured around any individual foreground lens. In practice, however, due to the relatively low mass of field lenses and the inherent randomness in intrinsic shape of background sources (the "shape noise"), the signal is impossible to measure on a galaxy by galaxy basis. However, by combining the signals of many individual lens measurements together (a technique known as "stacking"), the signal-to-noise ratio will improve, allowing one to determine a statistically significant signal, averaged over the entire lens set.

Scientific applications

Galaxy-galaxy lensing (like all other types of gravitational lensing) is used to measure several quantities pertaining to mass:

Mass density profiles

Using techniques similar to those in cluster-scale lensing, galaxy-galaxy lensing can provide information about the shape of mass density profiles, though these profiles correspond to galaxy-sized objects instead of larger clusters or groups. Given a high enough number density of background sources, a typical galaxy-galaxy mass density profile can cover a wide range of distances (from ~1 to ~100 effective radii). Since the effects of lensing are insensitive to the matter type, a galaxy-galaxy mass density profile can be used to probe a wide range of matter environments: from the central cores of galaxies where baryons dominate the total mass fraction, to the outer halos where dark matter is more prevalent.

Mass-to-light ratios

Comparing the measured mass to the luminosity (averaged over the entire galaxy stack) in a specific filter, galaxy-galaxy lensing can also provide insight into the mass to light ratios of field galaxies. Specifically, the quantity measured through lensing is the total (or virial) mass to light ratio – again due to the insensitivity of lensing to matter type. Assuming that luminous matter can trace dark matter, this quantity is of particular importance, since measuring the ratio of luminous (baryonic) matter to total matter can provide information regarding the overall ratio of baryonic to dark matter in the universe.

Galaxy mass evolution

Since the speed of light is finite, an observer on the Earth will see distant galaxies not as they look today, but rather as they appeared at some earlier time. By restricting the lens sample of a galaxy-galaxy lensing study to lie at only one particular redshift, it is possible to understand the mass properties of the field galaxies that existed during this earlier time. Comparing the results of several such redshift-restricted lensing studies (with each study encompassing a different redshift), one can begin to observe changes in the mass features of galaxies over a period of several epochs, leading towards a better understanding of the evolution of mass on the smallest cosmological scales.

Other mass trends

Lens redshift is not the only quantity of interest that can be varied when studying mass differences between galaxy populations, and often there are several parameters used when segregating objects into galaxy-galaxy lens stacks. Two widely used criteria are galaxy color and morphology, which act as tracers of (among other things) stellar population, galaxy age, and local mass environment. By separating lens galaxies based on these properties, and then further segregating samples based on redshift, it is possible to use galaxy-galaxy lensing to see how several different types of galaxies evolve through time.

Cosmic shear

The gravitational lensing by large-scale structure also produces an observable pattern of alignments in background galaxies, but this distortion is only ~0.1%-1% - much more subtle than cluster or galaxy-galaxy lensing. The thin lens approximation usually used in cluster and galaxy lensing does not always work in this regime, because structures can be elongated along the line of sight. Instead, the distortion can be derived by assuming that the deflection angle is always small (see Gravitational Lensing Formalism). As in the thin lens case, the effect can be written as a mapping from the unlensed angular position ${\displaystyle {\vec {\beta }}}$ to the lensed position ${\displaystyle {\vec {\theta }}}$. The Jacobian of the transform can be written as an integral over the gravitational potential ${\displaystyle \Phi }$ along the line of sight

${\displaystyle {\frac {\partial \beta _{i}}{\partial \theta _{j}}}=\delta _{ij}+\int _{0}^{r_{\infty }}drg(r){\frac {\partial ^{2}\Phi ({\vec {x}}(r))}{\partial x^{i}\partial x^{j}}}}$

where ${\displaystyle r}$ is the comoving distance, ${\displaystyle x^{i}}$ are the transverse distances, and

${\displaystyle g(r)=2r\int _{r}^{r_{\infty }}\left(1-{\frac {r^{\prime }}{r}}\right)W(r^{\prime })}$

is the lensing kernel, which defines the efficiency of lensing for a distribution of sources ${\displaystyle W(r)}$.

As in the thin-lens approximation, the Jacobian can be decomposed into shear and convergence terms.

Shear correlation functions

Because large-scale cosmological structures do not have a well-defined location, detecting cosmological gravitational lensing typically involves the computation of shear correlation functions, which measure the mean product of the shear at two points as a function of the distance between those points. Because there are two components of shear, three different correlation functions can be defined:

${\displaystyle \xi _{++}(\Delta \theta )=\langle \gamma _{+}({\vec {\theta }})\gamma _{+}({\vec {\theta }}+{\vec {\Delta \theta }})\rangle }$

${\displaystyle \xi _{\times \times }(\Delta \theta )=\langle \gamma _{\times }({\vec {\theta }})\gamma _{\times }({\vec {\theta }}+{\vec {\Delta \theta }})\rangle }$

${\displaystyle \xi _{\times +}(\Delta \theta )=\xi _{+\times }(\Delta \theta )=\langle \gamma _{+}({\vec {\theta }})\gamma _{\times }({\vec {\theta }}+{\vec {\Delta \theta }})\rangle }$

where ${\displaystyle \gamma _{+~}}$ is the component along or perpendicular to ${\displaystyle {\vec {\Delta \theta }}}$, and ${\displaystyle \gamma _{\times }}$ is the component at 45°. These correlation functions are typically computed by averaging over many pairs of galaxies. The last correlation function, ${\displaystyle \xi _{\times +}}$, is not affected at all by lensing, so measuring a value for this function that is inconsistent with zero is often interpreted as a sign of systematic error.

The functions ${\displaystyle \xi _{++~}}$ and ${\displaystyle \xi _{\times \times }}$ can be related to projections (integrals with certain weight functions) of the dark matter density correlation function, which can be predicted from theory for a cosmological model through its Fourier transform, the matter power spectrum.

Because they both depend on a single scalar density field, ${\displaystyle \xi _{++~}}$ and ${\displaystyle \xi _{\times \times }}$ are not independent, and they can be decomposed further into E-mode and B-mode correlation functions. In analogy with electric and magnetic fields, the E-mode field is curl-free and the B-mode field is divergence-free. Because gravitational lensing can only produce an E-mode field, the B-mode provides yet another test for systematic errors.

The E-mode correlation function is also known as the aperture mass variance

${\displaystyle \langle M_{ap}^{2}\rangle (\theta )=\int _{0}^{2\theta }{\frac {\phi d\phi }{\theta ^{2}}}\left[\xi _{++}(\phi )+\xi _{\times \times }(\phi )\right]T_{+}\left({\frac {\phi }{\theta }}\right)=\int _{0}^{2\theta }{\frac {\phi d\phi }{\theta ^{2}}}\left[\xi _{++}(\phi )-\xi _{\times \times }(\phi )\right]T_{-}\left({\frac {\phi }{\theta }}\right)}$

${\displaystyle T_{+}(x)=576\int _{0}^{\infty }{\frac {dt}{t^{3}}}J_{0}(xt)[J_{4}(t)]^{2}}$

${\displaystyle T_{-}(x)=576\int _{0}^{\infty }{\frac {dt}{t^{3}}}J_{4}(xt)[J_{4}(t)]^{2}}$

where ${\displaystyle J_{0}~}$ and ${\displaystyle J_{4}~}$ are Bessel Functions.

An exact decomposition thus requires knowledge of the shear correlation functions at zero separation, but an approximate decomposition is fairly insensitive to these values because the filters ${\displaystyle T_{+}~}$ and ${\displaystyle T_{-}~}$ are small near ${\displaystyle \theta =0~}$.

Weak lensing and cosmology

The ability of weak lensing to constrain the matter power spectrum makes it a potentially powerful probe of cosmological parameters, especially when combined with other observations such as the cosmic microwave background, supernovae, and galaxy surveys. Detecting the extremely faint cosmic shear signal requires averaging over many background galaxies, so surveys must be both deep and wide, and because these background galaxies are small, the image quality must be very good. Measuring the shear correlations at small scales also requires a high density of background objects (again requiring deep, high quality data), while measurements at large scales push for wider surveys.

While weak lensing of large-scale structure was discussed as early as 1967, due to the challenges mentioned above, it was not detected until more than 30 years later when large CCD cameras enabled surveys of the necessary size and quality. In 2000, four independent groups published the first detections of cosmic shear, and subsequent observations have started to put constraints on cosmological parameters (particularly the dark matter density ${\displaystyle \Omega _{m}~}$ and power spectrum amplitude ${\displaystyle \sigma _{8}~}$) that are competitive with other cosmological probes.

For current and future surveys, one goal is to use the redshifts of the background galaxies (often approximated using photometric redshifts) to divide the survey into multiple redshift bins. The low-redshift bins will only be lensed by structures very near to us, while the high-redshift bins will be lensed by structures over a wide range of redshift. This technique, dubbed "cosmic tomography", makes it possible to map out the 3D distribution of mass. Because the third dimension involves not only distance but cosmic time, tomographic weak lensing is sensitive not only to the matter power spectrum today, but also to its evolution over the history of the universe, and the expansion history of the universe during that time. This is a much more valuable cosmological probe, and many proposed experiments to measure the properties of dark energy and dark matter have focused on weak lensing, such as the Dark Energy Survey, Pan-STARRS, and Large Synoptic Survey Telescope.

Weak lensing also has an important effect on the Cosmic Microwave Background and diffuse 21cm line radiation. Even though there are no distinct resolved sources, perturbations on the origining surface are sheared in a similar way to galaxy weak lensing, resulting in changes to the power spectrum and statistics of the observed signal. Since the source plane for the CMB and high-redshift diffuse 21 cm are at higher redshift than resolved galaxies, the lensing effect probes cosmology at higher redshifts than galaxy lensing.

Negative weak lensing

Minimal coupling of general relativity with scalar fields allows solutions like traversable wormholes stabilized by exotic matter of negative energy density. Moreover, Modified Newtonian Dynamics as well as some bimetric theories of gravity consider invisible negative mass in cosmology as an alternative interpretation to dark matter, which classically has a positive mass.

As the presence of exotic matter would bend spacetime and light differently than positive mass, a Japanese team at the Hirosaki University proposed to use "negative" weak gravitational lensing related to such negative mass.

Instead of running statistical analysis on the distortion of galaxies based on the assumption of a positive weak lensing that usually reveals locations of positive mass "dark clusters", these researchers propose to locate "negative mass clumps" using negative weak lensing, i.e. where the deformation of galaxies is interpreted as being due to a diverging lensing effect producing radial distortions (similar to a concave lens instead of the classical azimuthal distortions of convex lenses similar to the image produced by a fisheye). Such negative mass clumps would be located elsewhere than assumed dark clusters, as they would reside at the center of observed cosmic voids located between galaxy filaments within the lacunar, web-like large-scale structure of the universe. Such test based on negative weak lensing could help to falsify cosmological models proposing exotic matter of negative mass as an alternative interpretation to dark matter.

Youth

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Youth 

A group of college women in the United States, 1973. The term adolescence is often considered synonymous with youth.

 

Young people dressed in casual wear attend Woodstock Festival of rock music, Poland, 2011

 

A group of youth in Sweden 2019.

Youth is the time of life when one is young, and often means the time between childhood and adulthood (maturity). It is also defined as "the appearance, freshness, vigor, spirit, etc., characteristic of one who is young". Its definitions of a specific age range varies, as youth is not defined chronologically as a stage that can be tied to specific age ranges; nor can its end point be linked to specific activities, such as taking unpaid work or having sexual relations.

Youth is an experience that may shape an individual's level of dependency, which can be marked in various ways according to different cultural perspectives. Personal experience is marked by an individual's cultural norms or traditions, while a youth's level of dependency means the extent to which they still rely on their family emotionally and economically.

Terminology and definitions

Students of a U.S. university do an outdoor class, where they discuss topics while walking.

 

Youth in Afghanistan

General

Around the world, the English terms youth, adolescent, teenager, kid, youngster and young person are interchanged, often meaning the same thing, but they are occasionally differentiated. Youth can be referred to as the time of life when one is young. This involves childhood, and the time of life which is neither childhood nor adulthood, but rather somewhere in between. Youth also identifies a particular mindset of attitude, as in "He is very youthful". For certain uses, such as employment statistics, the term also sometimes refers to individuals from the ages of 14 to 21. However, the term adolescence refers to a specific age range during a specific developmental period in a person's life, unlike youth which is a socially constructed category.

The United Nations defines youth as persons between the ages of roughly 15 and 24 with all UN statistics based on this range, the UN states education as a source for these statistics. The UN also recognizes that this varies without prejudice to other age groups listed by member states such as 18–30. A useful distinction within the UN itself can be made between teenagers (i.e. those between the ages of 13 and 19) and young adults (those between the ages of 20 and 29). While seeking to impose some uniformity on statistical approaches, the UN itself is aware of contradictions between approaches in its own statutes. Hence under the 15–24 definition (introduced in 1981) children are defined as those under the age of 14 (someone 13 and younger) while under the 1979 Convention on the Rights of the Child, those under the age of 18 are regarded as children. The UN also states they are aware that several definitions exist for youth within UN entities such as Youth Habitat 15–32, NCSL 12-24, and African Youth Charter 15–35.

On November 11, 2020, the State Duma of the Russian Federation approved a project to raise the cap on the age of young people from 30 to 35 years (the range now extending from 14 to 35 years).

Although linked to biological processes of development and aging, youth is also defined as a social position that reflects the meanings different cultures and societies give to individuals between childhood and adulthood. The term in itself when referred to in a manner of social position can be ambiguous when applied to someone of an older age with very low social position; potentially when still dependent on their guardians. Scholars argue that age-based definitions have not been consistent across cultures or times and that thus it is more accurate to focus on social processes in the transition to adult independence for defining youth.

"This world demands the qualities of youth: not a time of life but a state of mind, a temper of the will, a quality of imagination, a predominance of courage over timidity, of the appetite for adventure over the life of ease." – Robert Kennedy

Youth is the stage of constructing the self-concept. The self-concept of youth is influenced by variables such as peers, lifestyle, gender, and culture. It is a time of a person's life when their choices are most likely to affect their future.

Other definitions

Youth skateboarding in Mexico

 

Students of Peru discuss agricultural issues.

In much of sub-Saharan Africa, the term "youth" is associated with young men from 12 to 30 or 35 years of age. Youth in Nigeria includes all members of the Federal Republic of Nigeria aged 18–35. Many African girls experience youth as a brief interlude between the onset of puberty and marriage and motherhood. But in urban settings, poor women are often considered youth much longer, even if they bear children outside of marriage. Varying culturally, the gender constructions of youth in Latin America and Southeast Asia differ from those of sub-Saharan Africa. In Vietnam, widespread notions of youth are sociopolitical constructions for both sexes between the ages of 15 and 35.

In Brazil, the term youth refers to people of both sexes from 15 to 29 years old. This age bracket reflects the influence on Brazilian law of international organizations like the World Health Organization (WHO). It is also shaped by the notion of adolescence that has entered everyday life in Brazil through a discourse on children's rights.

The OECD defines youth as "those between 15 and 29 years of age".

August 12 was declared International Youth Day by the United Nations.

Youth rights

Main article: Youth rights

Children's rights cover all the rights that belong to children. When they grow up they are granted with new rights (like voting, consent, driving, etc.) and duties (criminal response, etc.). There are different minimum limits of age at which youth are not free, independent or legally competent to take some decisions or actions. Some of these limits are: voting age, age of candidacy, age of consent, age of majority, age of criminal responsibility, drinking age, driving age, etc. After youth reach these limits they are free to vote, have sexual intercourse, buy or consume alcoholic beverages or drive cars, etc.

Voting age

Main article: Voting age

Voting age is the minimum age established by law that a person must attain to be eligible to vote in a public election. Typically, the age is set at 18 years; however, ages as low as 16 and as high as 21 exist (see list below). Studies show that 21% of all 18-year-olds have experience with voting. This is an important right since, by voting, they can support politics selected by themselves and not only by people of older generations.

Age of candidacy

Main article: Age of candidacy

Age of candidacy is the minimum age at which a person can legally qualify to hold certain elected government offices. In many cases, it also determines the age at which a person may be eligible to stand for an election or be granted ballot access.

Age of consent

Main article: Age of consent

The age of consent is the age at which a person is considered legally competent to consent to sexual acts, and is thus the minimum age of a person with whom another person is legally permitted to engage in sexual activity. The distinguishing aspect of the age of consent laws is that the person below the minimum age is regarded as the victim, and their sex partner as the offender.

Defense of infancy

Main article: Defense of infancy

The defense of infancy is a form of defense known as an excuse so that defendants falling within the definition of an "infant" are excluded from criminal liability for their actions, if at the relevant time, they had not reached an age of criminal responsibility. This implies that children lack the judgment that comes with age and experience to be held criminally responsible. After reaching the initial age, there may be levels of responsibility dictated by age and the type of offense committed.

Drinking age

Main article: Drinking age

The legal drinking age is the age at which a person can consume or purchase alcoholic beverages. These laws cover a wide range of issues and behaviors, addressing when and where alcohol can be consumed. The minimum age alcohol can be legally consumed can be different from the age when it can be purchased in some countries. These laws vary among different countries and many laws have exemptions or special circumstances. Most laws apply only to drinking alcohol in public places, with alcohol consumption in the home being mostly unregulated (an exception being the UK, which has a minimum legal age of five for supervised consumption in private places). Some countries also have different age limits for different types of alcoholic drinks.

Driving age

Main article: Driving age

Driving age is the age at which a person can apply for a driver's license. Countries with the lowest driving ages (below 17) are Argentina, Australia, Canada, El Salvador, Iceland, Israel, Estonia, Macedonia, Malaysia, New Zealand, Norway, the Philippines, Russia, Saudi Arabia, Slovenia, Sweden, the United Kingdom (Mainland) and the United States. The Canadian province of Alberta and several U.S. states permit youth driving as low as 14. Niger has the highest minimum driving age in the world at 23. In India, driving is legal after getting a license at the age of 18.

Legal working age

Main article: Legal working age

See also: Right to work

The legal working age is the minimum age required by law for a person to work in each country or jurisdiction. The threshold of adulthood, or "the age of majority" as recognized or declared in law in most countries, has been set at age 18. Some types of labor are commonly prohibited even for those above the working age, if they have not reached the age of majority. Activities that are dangerous, harmful to the health or that may affect the morals of minors fall into this category.

Student rights in higher education

Main article: Student rights in higher education

Student rights are those rights, such as civil, constitutional, contractual and consumer rights, which regulate student rights and freedoms and allow students to make use of their educational investment. These include such things as the right to free speech and association, to due process, equality, autonomy, safety and privacy, and accountability in contracts and advertising, which regulate the treatment of students by teachers and administrators.

Smoking age

Main article: Smoking age

The smoking age is the minimum age a person can buy tobacco and/or smoke in public. Most countries regulate this law at the national level while at some point it is done by the state or province.

School and education

Main article: Schooling

Young people spend much of their lives in educational settings, and their experiences in schools, colleges and universities can shape much of their subsequent lives. Research shows that poverty and income affect the likelihood for the incompletion of high school. These factors also increase the likelihood for the youth to not go to a college or university. In the United States, 12.3 percent of young people ages 16 to 24 are disconnected, meaning they are neither in school nor working.

Health and mortality

Youths in South Africa partying

The leading causes of morbidity and mortality among youth and adults are due to certain health-risk behaviors. These behaviors are often established during youth and extend into adulthood. Since the risk behaviors in adulthood and youth are interrelated, problems in adulthood are preventable by influencing youth behavior.

A 2004 mortality study of youth (defined in this study as ages 10–24) mortality worldwide found that 97% of deaths occurred in low to middle-income countries, with the majority in southeast Asia and sub-Saharan Africa. Maternal conditions accounted for 15% of female deaths, while HIV/AIDS and tuberculosis were responsible for 11% of deaths; 14% of male and 5% of female deaths were attributed to traffic accidents, the largest cause overall. Violence accounted for 12% of male deaths. Suicide was the cause of 6% of all deaths.

The U.S. Centers for Disease Control and Prevention developed its Youth Risk Behavior Surveillance System (YRBSS) in 2003 to help assess risk behavior. YRBSS monitors six categories of priority health-risk behaviors among youth and young adults. These are behaviors that contribute to unintentional injuries and violence;

• tobacco, alcohol and other drug use;
• sexual behaviors that contribute to unintended pregnancy and sexually transmitted diseases (STDs), including human immunodeficiency virus (HIV) infection;
• unhealthy dietary behaviors;
• physical inactivity—plus overweight.

YRBSS includes a national school-based survey conducted by CDC as well as state and local school-based surveys conducted by education and health agencies.

Universal school-based interventions such as formal classroom curricula, behavioural management practices, role‐play, and goal‐setting may be effective in preventing tobacco use, alcohol use, illicit drug use, antisocial behaviour, and improving physical activity of young people.

Obesity

Main article: Childhood obesity

Obesity now affects one in five children in the United States, and is the most prevalent nutritional disease of children and adolescents in the United States. Although obesity-associated morbidities occur more frequently in adults, significant consequences of obesity as well as the antecedents of adult disease occur in obese children and adolescents.

Discrimination against overweight children begins early in childhood and becomes progressively institutionalized. Obese children may be taller than their non-overweight peers, in which case they are apt to be viewed as more mature. The inappropriate expectations that result may have an adverse effect on their socialization.

Many of the cardiovascular consequences that characterize adult-onset obesity are preceded by abnormalities that begin in childhood. Hyperlipidemia, hypertension, and abnormal glucose tolerance occur with increased frequency in obese children and adolescents. The relationship of cardiovascular risk factors to visceral fat independent of total body fat remains unclear. Sleep apnea, pseudotumor cerebri, and Blount's disease represent major sources of morbidity for which rapid and sustained weight reduction is essential. Although several periods of increased risk appear in childhood, it is not clear whether obesity with onset early in childhood carries a greater risk of adult morbidity and mortality.

Bullying

Bullying among school-aged youth is increasingly being recognized as an important problem affecting well-being and social functioning. While a certain amount of conflict and harassment is typical of youth peer relations, bullying presents a potentially more serious threat to healthy youth development. The definition of bullying is widely agreed on in literature on bullying.

Bullying often happens in schools

The majority of research on bullying has been conducted in Europe and Australia. Considerable variability among countries in the prevalence of bullying has been reported. In an international survey of adolescent health-related behaviors, the percentage of students who reported being bullied at least once during the current term ranged from a low of 15% to 20% in some countries to a high of 70% in others. Of particular concern is frequent bullying, typically defined as bullying that occurs once a week or more. The prevalence of frequent bullying reported internationally ranges from a low of 1.9% among one Irish sample to a high of 19% in a Malta study.

Research examining characteristics of youth involved in bullying has consistently found that both bullies and those bullied demonstrate poorer psychosocial functioning than their non-involved peers. Youth who bully others tend to demonstrate higher levels of conduct problems and dislike of school, whereas youth who are bullied generally show higher levels of insecurity, anxiety, depression, loneliness, unhappiness, physical and mental symptoms, and low self-esteem. Males who are bullied also tend to be physically weaker than males in general. The few studies that have examined the characteristics of youth who both bully and are bullied found that these individuals exhibit the poorest psychosocial functioning overall.

See also: School bullying

Sexual health and politics

General

Globalization and transnational flows have had tangible effects on sexual relations, identities, and subjectivities. In the wake of an increasingly globalized world order under waning Western dominance, within ideologies of modernity, civilization, and programs for social improvement, discourses on population control, 'safe sex', and 'sexual rights'. Sex education programmes grounded in evidence-based approaches are a cornerstone in reducing adolescent sexual risk behaviours and promoting sexual health. In addition to providing accurate information about consequences of Sexually transmitted disease or STIs and early pregnancy, such programmes build life skills for interpersonal communication and decision making. Such programmes are most commonly implemented in schools, which reach large numbers of teenagers in areas where school enrollment rates are high. However, since not all young people are in school, sex education programmes have also been implemented in clinics, juvenile detention centers and youth-oriented community agencies. Notably, some programmes have been found to reduce risky sexual behaviours when implemented in both school and community settings with only minor modifications to the curricula.

Philippines

The Sangguniang Kabataan ("Youth Council" in English), commonly known as SK, was a youth council in each barangay (village or district) in the Philippines, before being put "on hold", but not quite abolished, prior to the 2013 barangay elections. The council represented teenagers from 15 to 17 years old who have resided in their barangay for at least six months and registered to vote. It was the local youth legislature in the village and therefore led the local youth program and projects of the government. The Sangguniang Kabataan was an offshoot of the KB or the Kabataang Barangay (Village Youth) which was abolished when the Local Government Code of 1991 was enacted.

In the Global South

The vast majority of young people live in developing countries: according to the United Nations, globally around 85 per cent of 15- to 24-year-olds live in developing countries, a figure projected to grow 89.5 per cent by 2025. Moreover, this majority are extremely diverse: some live in rural areas but many inhabit the overcrowded metropolises of India, Mongolia and other parts of Asia and in South America, some live traditional lives in tribal societies, while others participate in global youth culture in ghetto contexts.

Many young lives in developing countries are defined by poverty, some suffer from famine and a lack of clean water, while involvement in armed conflict is all common. Health problems are rife, especially due to the prevalence of HIV/AIDS in certain regions. The United Nations estimates that 200 million young people live in poverty, 130 million are illiterate and 10 million live with HIV/AIDS.