How Do Galaxies Form?


The Hubble telescope has shown us incredible sights, including an estimated 100 billion galaxies. That’s definitely not how many are out there, though. Once we have more powerful telescopes, we’ll probably see about 200 billion. So how did all those galaxies get there? How do galaxies form?


1. We don’t know for sure.
Most galaxies formed when the universe was very, very young. For this reason, we don’t really have examples of how galaxies form and so we have to make an educated guess based on computer models.

2. Gas clouds form high mass stars.
Based on simulations, what we think happened is that clouds of gas and dust that were floating out in the early universe began to come together as high-mass stars. When enough of these stars pull together, they become protogalaxies and star collecting gas and dust.


3. As more stars cluster together, the gravity strengthens.
Galaxies are held together by gravity, and it takes a lot of stars to create enough gravity for an entire galaxy to coalesce. Eventually, the gravity becomes so strong that the galaxy begins to spin around a common center of mass. And tada! A fully formed galaxy is born. At least, that’s the basics of what we think happens.
  • Like
Reactions: ManosMyron
May 12, 2020
Visit site
galaxieswere born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form. The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies.
Replying to: SaraRayne and ManosMyron on Galactic Formation

In order to see how galaxies formed, we need to see how galaxies have changed over time and we need to find galaxies of different ages so we can see how galaxies change over time. We can utilize our own Milky Way and the Local Group of galaxies as examples of older galaxies, but we need a sample of young galaxies for comparison and to find young galaxies, we need to identify very distant galaxies. The reason distant galaxies = young galaxies is the finite speed of light. If you observe an object 1 million light years away, you are not seeing it as it is today. The light you currently see left the object 1 million years ago. This phenomenon is called lookback time. So, if you want to find a galaxy 5 billion years younger than the Milky Way, you should search for galaxies 5 billion light years away. Then, you can compare those galaxies to the ones you find 10 billion light years away, because those will appear as they were 10 billion years ago.

We can directly observe how galaxies appeared when they were forming if we can find galaxies at very large lookback times. In recent years, astronomers have been using the technique of observing deep fields (like the the Hubble Ultra Deep Field) to pursue the most distant galaxies in the universe which we can observe. This ultra deep field, in particular, has helped answer the question: “How did galaxies look billions of years ago?” The answer appears to be that when galaxies were young, they looked very irregular and ragged. Galaxies with spiral arms like the Milky Way did not appear until about 10 billion years ago. We think that galaxies apparently formed from the bottom up; that is, more than 10 billion years ago, small, irregularly shaped sub-galaxies appear to have collided and merged, leading to the formation of the large spiral and barred spiral galaxies that we see today. Although the Milky Way continues to form new stars today, primarily in its spiral arms, the star formation rates in these sub-galaxies were much higher. Observations suggest that the peak of star formation occurred about 8 billion years ago.

hubble ultra deep field.jpg
Portion of Hubble Ultra Deep Field image, showing young galaxies. Credit: Hubblesite

By comparing local galaxies to distant galaxies, as they appear in the above Hubble Ultra Deep Field Image, and supplementing these observations with computer simulations of the early universe, astronomers believe that galaxies form in roughly the following steps:
  1. The first objects are sub-galaxy sized "pieces."
  2. Several of these pieces coalesce to form a larger mass object.
  3. The gas in the larger galaxy can collapse, increasing the rotation speed of the galaxy.
  4. Stars will rapidly form inside this disk, and their orbits will sort into the familiar spiral structure.
  5. Disk galaxies will continue to evolve by the various interaction processes we saw previously, and major mergers will create elliptical galaxies.
In this general prescription for the evolution of galaxies, we did not fit the Active Galactic Nucleus (AGN) into the scenario. The AGN phase appears to be a short phase in the overall lifetime of a galaxy, and it occurs when the Super Massive Black Hole (SMBH) in the core of that galaxy has enough fuel to power the enormous luminosities these objects emit. Again, using lookback time, we see that quasars* are most numerous about 10 billion years ago. So, the quasar phase appears to be an early phase that perhaps most galaxies went through before settling down as normal galaxies.

Additionally, we must examine the impact of Cold Dark Matter (CDM) on galactic formation.
Semi-Analytic Models (SAMs) are still the best way to understand the formation of galaxies and clusters within the cosmic web dark matter gravitational skeleton, because they allow comparison of variant models of star and supermassive black hole formation and feedback. This touches on the current state of the art in semi-analytic models, and describes the successes and challenges for the best current CDM models of the roles of baryonic physics and supermassive black holes in the formation of galaxies.

Dark matter preserved the primordial fluctuations in cosmological density on galaxy scales that were wiped out in baryonic matter by momentum transport (viscosity) as radiation decoupled from baryons in the first few hundred thousand years after the big bang. The growth of dark matter halos started early enough to result in the formation of galaxies that we see even at high redshifts z > 6. Dark matter halos provide most of the gravitation within which stable structures formed in the universe. In more recent epochs, dark matter halos preserve these galaxies, groups, and clusters as the dark energy tears apart unbound structures and expands the space between bound structures such as the Local Group of galaxies. Thus we owe our existence and future to dark matter.

Cold dark matter theory including cosmic inflation has become the basis for the standard modern CDM cosmology, which is favored by analysis of the available cosmic microwave background data and large scale structure data over even more complicated variant theories having additional parameters. Most of the cosmological density is nonbaryonic dark matter (about 23%) and dark energy (about 72%), with normal baryonic matter making up only about 4.6% and the visible baryons only about 0.5% of the cosmic density. The fact that dark energy and dark matter are dominant suggests a popular name for the modern standard cosmology: the ―double dark matter theory as Nancy Abrams and Joel R. Primack proposed in their recent book about modern cosmology.

Optical (dots) and radio (triangles) rotation curve data for the Andromeda galaxy M31. superimposed on the M31 image from the Palomar Sky Survey (from Vera Rubin)

The physical nature of dark matter remains to be discovered. The two most popular ideas concerning the identity of the dark matter particles remain the lightest supersymmetric partner particle, also called supersymmetric weakly interacting massive particles (WIMPs), and the cosmological axion. These are the two dark matter candidate particles that are best motivated in the sense that they are favored by other considerations of elementary particle theory.

Supersymmetry remains the best idea for going beyond the standard model of particle physics. It allows control of vacuum energy and of otherwise unrenormalizable gravitational interactions, and thus may allow gravity to be combined with the electroweak and strong interactions in superstring theory. Supersymmetry also allows for grand unification of the electroweak and strong interactions, and naturally explains how the electroweak scale could be so much smaller than the grand unification or Planck scales (thus solving the ―gauge hierarchy problem). It thus leads to the expectation that the supersymmetric WIMP mass will be in the range of about 100 to about 1000 GeV.

Axions** remain the best solution to the CP problem of SU(3) gauge theory of strong interactions, although it is possible that the axion exists and solves the strong CP problem but makes only a negligible contribution to the dark matter density.

Many other particles have been proposed as possible dark matter candidates, even within the context of supersymmetry. An exciting prospect in the next few years is that experimental and astronomical data may point toward specific properties of the dark matter particles, and may even enable us to discover their identity. There are good opportunities for detecting the dark matter particles in deep underground experiments, producing them at the Large Hadron Collider at CERN, detecting their annihilation products, and exploring the possibility that the dark matter is warm by studying small scale structure.

In order to get order-of-magnitude better constraints than presently available, and a possible detection of non-cosmological-constant dark energy, better instruments will probably be required both on the ground and in space, according to the Dark Energy Task Force. The National Academy Beyond Einstein report recommended the Joint Dark Energy Mission (JDEM) ( See: as the first Beyond Einstein mission. It also recommended that JDEM be conceived as a dual-purpose mission, collecting a wide range of data that will shed light on galaxy formation and evolution as well as on the nature dark energy.

* Quasar stands for quasi-stellar radio source. Quasars got that name because they looked starlike when astronomers first began to notice them in the late 1950s and early 60s. But quasars aren’t stars. They’re now known as young galaxies, located at vast distances from us, with their numbers increasing towards the edge of the visible universe. How can they be so far away and yet still visible? The answer is that quasars are extremely bright, up to 1,000 times brighter than our Milky Way galaxy. We know, therefore, that they’re highly active, emitting staggering amounts of radiation across the entire electromagnetic spectrum.

Because they’re far away, we’re seeing these objects as they were when our universe was young. The oldest quasar, currently, is J0313-1806. Its distance has been measured as 13.03 billion light-years, and therefore we see it as it was just 670 million years after the Big Bang.

Astronomers now believe that quasars are the extremely luminous centers of galaxies in their infancy. After decades of intense study, we have another term for these objects: a quasar is a type of active galactic nucleus, or AGN. There are actually many different types of AGNs, each with their own tale to tell. The intense radiation released by an AGN is thought to be powered by a super-massive black hole at its center. The radiation is emitted when material in the accretion disk surrounding the black hole is superheated to millions of degrees by the intense tidal friction generated by the particles of dust, gas and other matter in the disk, sometimes entire stars, colliding countless times with each other.

The inward spiral of matter in a supermassive black hole’s accretion disk – that is, at the center of a quasar – is the result of particles colliding and bouncing against each other and losing momentum. That material came from the enormous clouds of gas, mainly consisting of molecular hydrogen, which filled the universe in the era shortly after the Big Bang.

As matter in a quasar/black hole’s accretion disk heats up, it generates radio waves, X-rays, ultraviolet and visible light. The quasar becomes so bright that it’s able to outshine entire galaxies. But remember that quasars are so far away from us that we only observe the active nucleus, or core, of the galaxy in which they reside. We see nothing of the galaxy apart from its bright center. It’s like seeing a distant headlight at night: you have no idea of which type of career truck you are looking at, as everything apart from the headlight is in darkness.

Quasars can emit up to a thousand times the energy of the combined luminosity of the 200 billion or so stars in our own Milky Way galaxy. A typical quasar can be 27 trillion times brighter than our sun!

Astronomers believe that most, if not all, large galaxies went through a so-called “quasar phase” in their youth, soon after their formation. If so, they subsided in brightness when they ran out of the steady stream of matter needed to feed the accretion disk surrounding their supermassive black holes. After this epoch, galaxies settled into quiescence, their central black holes starved of that steady flow of material to feed on. The black hole at the center of our own galaxy has been seen to flare up briefly, however, as passing material strays into it, releasing radio waves and X-rays. It’s conceivable that entire stars or even clusters of stars could be torn apart and consumed as they cross a black hole’s event horizon, the point of no return.

When these massive visible jets of energy are perpendicular to our view, we see a radio galaxy. If they’re at an angle, we see a quasar. And when we’re staring right down the barrel of the jet, that’s a blazar. It’s the same object, seen from three different perspectives.

It must be pointed out, however, that our knowledge of galaxy evolution – from youthful quasar to quiescent middle-aged galaxy – is far from complete. Galaxies often provide us with exceptions, and as an example we need look no further than our own Milky Way. We now know that some 3.5 million years ago there was a gigantic explosion known as a Seyfert flare at the center of our galaxy. It was apparently centered on Sagittarius A*, the Milky Way’s own central supermassive black hole, producing two huge lobes of superheated plasma extending some 25,000 light years from the north and south galactic poles. These huge lobes are called Fermi bubbles and are still visible today at gamma and X-ray wavelengths (very high frequency electromagnetic emissions).

** Axions - The axion is a pseudo-Nambu-Goldstone boson. It appears after the spontaneous breaking of the Peccei-Quinn symmetry***, which was proposed to solve the strong-CP (charge conjugation parity symmetry) problem. Other pseudo-Nambu-Goldstone bosons, postulated in some extensions of the standard model of particle physics, are called axion-like particles (ALPs) if they share certain characteristics with the axion, in particular a coupling to two photons. Thus far, axion and ALP searches have been unsuccessful, indicating that their couplings have to be extremely weak. However, axions and ALPs could be responsible for some observable effects in astrophysics and cosmology, which can also be exploited to constrain the parameter space of these particles. We focus on limits coming from cosmology, which is an optimal field for studying axions and ALPs. In particular, we first investigate the possibility of a primordial population of axions and ALPs arising during the earliest epochs of the universe. The importance of this analysis lies on the fact that axions and ALPs are ideal dark matter candidates because of their faint interactions and their peculiar production mechanisms. Finally, we consider the consequences of the decay of such a population on specific cosmological observables, namely the photon spectrum of galaxies, the cosmic microwave background, the effective number of neutrino species, and the abundance of primordial elements. Our bounds constitute the most stringent probes of early decays and exclude a part of the ALP parameter space that is otherwise very difficult to test experimentally.
See: ;

*** The proximity of the Peccei-Quinn scale to the scale of supersymmetry breaking in models of pure gravity mediation hints at a common dynamical origin of these two scales. To demonstrate how to make such a connection manifest, we embed the Peccei-Quinn mechanism into the vector-like model of dynamical supersymmetry breaking a la IYIT. Here, we rely on the anomaly-free discrete Z4R symmetry required in models of pure gravity mediation to solve the mu problem to protect the Peccei-Quinn symmetry from the dangerous effect of higher-dimensional operators. This results in a rich phenomenology featuring a QCD axion with a decay constant of O(10^10) GeV and mixed WIMP/axion dark matter. In addition, exactly five pairs of extra 5 and 5* matter multiplets, directly coupled to the supersymmetry breaking sector and with masses close to the gravitino mass, m3/2 ~ 100 TeV, are needed to cancel the Z4R anomalies.





Thus, from pure mechanics, we have ventured into the dark matter and dark energy as galactic building blocks over time as found and charted by Vera Rubin among others. Galaxies change over time and the farther back we look we can see simpler galactic models and see them form in their five main steps. Again, I am astounded to realize that I am star stuff, as Carl Sagan termed us all, looking out on the universe from whence we all came and I can see beauty in every direction in every age of development.
Hartmann 352
Last edited:
Jul 26, 2021
Matter exerts gravitational attraction to other matter. It also appears that dark matter behaves similarly although we can't detect it with light. These gravitational forces cause matter to clump together as its kinetic energy permits, which is a galaxy.
Jul 29, 2021
Visit site
It might be helpful to take a glimpse into Galaxy formation

These are key (past-future) steps of galaxies exploration.

1. The first key idea dates to 1916 when Einstein developed his General Theory of Relativity. The simplest assumption to make was that if you viewed the contents of the universe with sufficiently poor vision, it would appear roughly the same everywhere and in every direction.

2. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe. Key factors should be considered, while trying to get a hint, how many it will find more, for instance, in the nearest 50 years. After Hubble data analysis astronomers came to the surprising conclusion that there are at least 10 times more galaxies in the observable universe than previously thought. Galaxies are not evenly distributed throughout the universe's history. One of the most fundamental questions in astronomy is that of just how many galaxies the universe contains. The landmark Hubble Deep Field, taken in the mid-1990s, gave the first real insight into the universe's galaxy population. Subsequent sensitive observations such as Hubble's Ultra Deep Field revealed a myriad of faint galaxies. This led to an estimate that the observable universe contained about 200 billion galaxies. The new research shows that this estimate was far inaccurate. This led to the surprising conclusion that in order for the numbers of galaxies we now see and their masses to add up, there must be a further 90 percent of galaxies in the observable universe that are too faint and too far away to be seen with present-day telescopes.
It might be helpful to take a glimpse into Galaxy formation

These are key (past-future) steps of galaxies exploration.

1. The first key idea dates to 1916 when Einstein developed his General Theory of Relativity. The simplest assumption to make was that if you viewed the contents of the universe with sufficiently poor vision, it would appear roughly the same everywhere and in every direction.

2. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe. Key factors should be considered, while trying to get a hint, how many it will find more, for instance, in the nearest 50 years. After Hubble data analysis astronomers came to the surprising conclusion that there are at least 10 times more galaxies in the observable universe than previously thought. Galaxies are not evenly distributed throughout the universe's history. One of the most fundamental questions in astronomy is that of just how many galaxies the universe contains. The landmark Hubble Deep Field, taken in the mid-1990s, gave the first real insight into the universe's galaxy population. Subsequent sensitive observations such as Hubble's Ultra Deep Field revealed a myriad of faint galaxies. This led to an estimate that the observable universe contained about 200 billion galaxies. The new research shows that this estimate was far inaccurate. This led to the surprising conclusion that in order for the numbers of galaxies we now see and their masses to add up, there must be a further 90 percent of galaxies in the observable universe that are too faint and too far away to be seen with present-day telescopes.

Larliss, the speed of each individual distant galaxy with respect to us will increase as time increases. If we assume that this acceleration continues indefinitely, as shown by Saul Perlmutter, Martin Riess and others*, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and these galaxies is stretching so rapidly that any light they emit after that point will never be able to reach us.

As time passes (billions of years in the future), we will see these galaxies freeze and fade, never to be heard from again. Furthermore, as more and more galaxies accelerate past the speed of light, any light that they emit after a certain point will also not be able to reach us, and they too will freeze and fade away. Eventually, we will be left with a universe that is mostly invisible and dark, with only the light from a few, very nearby galaxies (whose motions are strongly affected by local gravitational interaction) to keep us company at night. If the human race survives that far in the distant future, that is.

* Saul Perlmutter won the 2011 Nobel Prize in Physics “for the discovery of the accelerating expansion of the universe through observations of distant Type IA supernovae.” Perlmutter heads the international Supernova Cosmology Project, which pioneered the methods used to discover the accelerating expansion of the universe, and he has been a leader in studies to determine the nature of dark energy.
Last edited:
  • Like
Reactions: Lariliss
The new Webb telescope(JWST) detector is suppose to be able to see thru dust clouds. I hope they can mapped those clouds also. The Milky Way might be full of dust. And as we get into the center, the combined stars winds should grow denser too. We might also have of lot of cooler invisible mass.

I hope there are no more delays. Once in orbit of L1(I think), it's suppose to take 90 days to calibrate. It might be well into next year before we have images.
While an operational Webb telescope still lies years in the future and may see through dust clouds, astronomers are using data today from NASA’s Spitzer Space Telescope* and have searched for dust-hidden supernovae in the nuclear regions of 40 luminous and ultra-luminous infrared galaxies within 200 Mpc (652 million light-years) of earth.

This image shows galaxy Arp 148, captured by NASA’s Spitzer Space Telescope and the NASA/ESA Hubble Space Telescope; specially processed Spitzer data is shown inside the white circle, revealing infrared light from a supernova hidden by dust; this is one five hidden supernovae documented by Fox et al. Image credit: NASA / JPL-Caltech.

“Our results with Spitzer show that the optical surveys we’ve long relied on for detecting supernovae miss up to half of the stellar explosions happening out there in the Universe,” said Dr. Ori Fox, an astronomer at the Space Telescope Science Institute.

“It’s very good news that the number of supernovae we’re seeing with Spitzer is statistically consistent with theoretical predictions.”

For the study, Dr. Fox and colleagues selected a local set of 40 dust-choked galaxies, known as luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs, respectively).

The dust in LIRGs and ULIRGs absorbs optical light from objects like supernovae but allows infrared light from these same objects to pass through unobstructed for telescopes like Spitzer to detect because, although the dust renders the galaxies opaque, the dust particles reflect the absorbed light in the far infra-red frequencies.

The astronomers detected nine supernovae, five of which were not discovered by optical surveys.

“It’s a testament to Spitzer’s discovery potential that the telescope was able to pick up the signal of hidden supernovae from these dusty galaxies,” Dr. Fox said.

The types of supernovae detected by this team are known as core-collapse supernovae, involving giant stars with at least eight times the mass of the Sun.

As they grow old and their cores fill with iron, the big stars can no longer produce enough energy to withstand their own gravity, and their cores collapse, suddenly and catastrophically.

The intense pressures and temperatures produced during the rapid cave-in forms new chemical elements via nuclear fusion.

The collapsing stars ultimately rebound off their ultra-dense cores, blowing themselves to smithereens and scattering those elements throughout space.

“If you have a handle on how many stars are forming, then you can predict how many stars will explode,” Dr. Fox said.

“Or, vice versa, if you have a handle on how many stars are exploding, you can predict how many stars are forming. Understanding that relationship is critical for many areas of study in astrophysics.”

The results were published in the Monthly Notices of the Royal Astronomical Society.

See: Ori D. Fox et al. 2021. A Spitzer survey for dust-obscured supernovae. MNRAS506 (3): 4199-4209; doi: 10.1093/mnras/stab1740

* Spitzer Space Telescope was launched at 05:35:39 UT Aug. 25, 2003, on a Delta II Heavy (in a two-stage Delta 7925H configuration) inserted the second stage and payload. The initial orbit was 103 × 104 miles (166 × 167 kilometers) at 31.5 degrees. The second stage ignited again at 06:13 UT Aug. 25, 2003, sending both the second stage and the observatory into a hyperbolic orbit. By Sept. 3, the telescope was in an Earth-trailing orbit around the Sun.

The telescope’s dust cover was ejected Aug. 29 and its aperture door opened the next day. In this orbit, at 0.996 × 1.019 AU*, Earth doesn’t hinder observation of potential targets.

On Dec. 18, 2003, the SIRTF was renamed the Spitzer Space Telescope in honor of Lyman S. Spitzer, Jr. (1914-1997), one of the first people to propose the idea of using telescopes in space.

One of the early successes of the mission (in 2005) was to capture direct light from extrasolar planets for the first time.

Many other findings followed in the subsequent four years, including seeing light from the earliest objects in the universe, mapping the weather on an extrasolar planet for the first time, finding water vapor on another extrasolar planet, and identifying a new ring (the Phoebe ring) around Saturn.

The observatory has worked far longer than expected, but its supply of liquid helium finally depleted at 22:11 UT May 15, 2009, nearly six years after launch. At that point, mission scientists reconfigured the mission as the Spitzer Warm Mission, which would use the two shortest-wavelength modules of the infrared array camera (IRAC), which did not require the cryogenic helium to operate, for future observations.

More discoveries followed. In August 2010, data from Spitzer revealed the identification of the first carbon-rich planet (known as WASP-12b) orbiting a star. In October 2012, astronomers announced that data from the observatory had allowed more precise measurement of the Hubble constant, the rate at which the universe is stretching apart.

The following year, Spitzer celebrated 10 full years of operation in space and continued operation of its two instruments which, in August 2014, observed an eruption of dust around a star (NGC 2547-ID8), possibly caused by a collision of large asteroids. Such impacts are thought to lead to the formation of planets.

Continuing discoveries based on results from Spitzer (as well as data integrated with information from other space-based observatories such as Swift) were announced in April 2015 (discovery of one of the most distant planets ever identified, about 13,000 light-years from Earth) and in March 2016 (discovery of the most remote galaxy ever detected, a high-redshift galaxy known as GN-z11). The latter was detected as part of the Frontiers Field project that combines the power of Spitzer, Hubble and Chandra.

In August 2016, mission planners at NASA's Jet Propulsion Laboratory (JPL) announced a new phase of the mission known as “Spitzer Beyond,” leveraged on a two-and-a-half-year mission extension granted by NASA earlier in the year.

Because the distance between Spitzer and Earth has widened over time, the telescope’s antenna must be pointed at higher angles toward the Sun to communicate with Earth. As a result, parts of the spacecraft will experience increasing amounts of heat. Simultaneously, its solar panels will be pointed away from the Sun in this configuration, thus putting onboard batteries under more stress. These challenges will be a part of the Spitzer Beyond phase.

In February 2017, NASA announced that Spitzer had revealed the first known system of seven Earth-size planets around a single star. Three of the planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water. The discovery set a record for the greatest number of habitable-zone planets found around a single star outside our solar system.

In October 2017, NASA announced that it was seeking information from potential funders who might be able to support operation of the telescope after NASA funding runs out.

In August 2018, Spitzer marked 15 years in operation.

See: Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.



* AU: Astronomical Unit (AU) is the average distance between Earth and the Sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our Solar System. For example, the planet Mercury is about 1/3 of an AU from the sun, while the farthest planet, Pluto, is about 40 AU from the sun (that's 40 times as far away from the Sun as Earth is).


It is interesting to see that a reexamination of the old Spitzer satellite data has yielded a growing number of heretofore unknown supernovae in observable and some newly discovered dusty galaxies. Again, it seems very cool that this number of recently found supernovae puts the once growing concern about the "observable supernovae gap" to bed. All it took was a minute exploration of existing data and "poof" we are again on the correct statistical path in identifying and classifying supernovae in the visible universe.
  • Like
Reactions: Lariliss
Sep 10, 2021
Visit site
Galaxies were born soon after stars. It is believed that the first luminaries flashed no later than 150 million years after the Big Bang. According to model calculations, within a hundred million years after the Big Bang, clouds of dark matter the size of the current solar system formed in space.
Syksy Räsänen's main research topics at the moment are the effect of cosmological structure formation on the expansion of the universe, on light propagation, and Higgs inflation. He's interested in the backreaction conjecture, according to which structure formation would lead to the observed larger expansion rate and longer distances without the need for dark energy or modified gravity. Other research topics past and present include the cosmic microwave background, dark energy, magnetogenesis, tests of homogeneity and isotropy, primordial black holes, string gas cosmology and the ekpyrotic scenario. Syksy is a member of the Euclid consortium.


Another look at redshift drift and the backreaction conjecture
by S. M. Koksbang
30 September 2019
Earlier studies have conjectured that redshift drift is described by spatially averaged quantities and thus becomes positive if the average expansion of the Universe accelerates. This conclusion is reevaluated here by considering exact light propagation in a simple toy-model with average accelerated expansion. The toy-model and light propagation setup is explicitly designed for concordance between spatial averages and averages along light rays. While it is verified that redshift-distance relations are well described by average quantities in this setup, it is found that the redshift drift is not. Specifically, the redshift drift is negative despite the on-average late-time accelerated expansion of the model. This result implies that measuring redshift drift signals at low redshifts gives the potential for directly falsifying the backreaction conjecture. However, the results are based on a toy-model so it is in principle possible that the result is an artifact and that redshift drift is in reality well described by spatially averaged quantities. The result therefore highlights the importance of developing \emph{exact} solutions to the Einstein equations which exhibit average accelerated expansion without local expansion so that the relation between spatial averages and observations can be firmly established.
A toy-model of disjoint FLRW regions (positively curved and void) was constructed in a way that resulted in late-time average accelerated expansion without any local accelerated expansion due to e.g. dark energy. Light propagation in the model was studied both by exact ray tracing through an ensemble of disjoint regions and through schemes proposed in the literature for describing on-average light propagation in statistically homogeneous and isotropic spacetimes through spatially averaged quantities. The model was specifically de- signed to fulfill the assumptions leading to one of these schemes (the covariant scheme). The covariant scheme was the only one that gave a good description of the exact redshift-distance relation. Therefore, only this scheme was used to study redshift drift in the model. According to this scheme, the redshift drift should be positive at low redshifts in the studied model due to the late-time average accelerated expansion. However, the exact redshift drift computed along light rays with different observers was non-positive at all redshifts. This result must be considered with caution as it was obtained by studying a toy-model that was specifically not an exact solution to the Einstein equations. It is striking though, that the studied model was specifically designed for the assumptions of the covariant scheme to be fulfilled and that the redshift and redshift-distance relation were both well reproduced by the covariant scheme, while only the redshift drift was not.


If the result obtained was valid in general for models exhibiting non-negligible backreaction, it gives a possible method to attempt a falsification of the backreaction conjecture which proposes that the apparent late-time accelerated expansion of the universe is an effect of averaging and not an actual, local acceleration.

It is therefore important that exact solutions to the Einstein equations are developed so that it can be firmly established whether or not redshift drift becomes positive due to the average accelerated expansion.

I can see why Saul Perlmutter's* et al use of Type Ia Supernovae as standard candles in the use of establishing the accelerating expansion of the universe is never mentioned in the explanation of the backreaction conjecture, because it is, after all, a proposed effect of averaging and not a real, local acceleration.