# What is the characteristic time of the evaporation of the galaxies?

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For a star, to leave its galaxy, it requires probably a 3-body near-collision at the edge of the galaxy. It is unlikely, but possible. And, if a star once somehow got the required velocity to escape the galaxy, then it will escape and never comes back.

Thus, galaxies should have a characteristic time of their evaporation, what could be calculated. I suspect, this time is probably longer, than other related processes (expansion of the Universe, end of the age of the stars, etc), but it exists can it can be probably calculated (most likely, by numeric simulations).

It probably also depends on the size and star density of the galaxy.

Was it calculated? How big is it?

The standard treatment can be found in (Binney & Tremaine 2008), but see also (Adams & Laughlin 1997) for a good treatment.

The overall timescale for galactic evaporation is $$au_{evap}= 100 au_{relax}sim 10^{19}$$ years.

The relaxation timescale $$au_{relax}=frac{R}{v}frac{N}{12 ln(N/2)},$$ where $$R$$ is the size of the system, $$v$$ is the typical random velocity, and $$N$$ is the total number of stars. This corresponds to the time it takes to completely randomize the velocity of a star by interactions with other stars.

## What is the characteristic time of the evaporation of the galaxies? - Astronomy

What causes a fundamental particle (e.g. one of the heavier quarks) to "decay" into other fundamental particles, and where do these new particles come from if they are not part of the original particle?

It seems to me that for a particle to decay, it must have some sort of internal or external force acting on it, but how is this possible if all forces are borne by other fundamental particles? And as to the new particles formed, I am perplexed by their ability to spring into existence after the original particle ceases to exist. I am a high school student with some knowledge of physics, but I have only recently begun reading about particle physics.

In a sense, particles will decay because they are lazy: they want to be in the lowest possible energy state they can reach. So, if the decay products have lower energy than the initial particle, the decay can happen spontaneously. That means that the particle can be sitting in the middle of nowhere with absolutely no forces acting on it, and it will still decay. Though it is not possible to predict the exact time at which the decay will happen, the particles have a characteristic lifetime which they typically live (this is very close to their half-life, if you've heard of that term before).

As an example, a neutron is slightly heavier than a proton, so it has slightly more energy than the latter. It turns out that left alone, a free neutron (one that isn't bound in a nucleus) will spontaneously decay into a proton, and electron and a neutrino (this is called "beta decay"). The characteristic time for the decay to occur is about 15 minutes.

Finally, what kinds of particles can decay in this way? It turns out that any particles that are composites of fundamental particles (like protons, neutrons, and atoms full of protons and neutrons) can decay in this manner. As for fundamental particles themselves, an electron for example can't spontaneously change into anything else in the same way that a neutron decays. The quarks which you mention are a more difficult case, because we don't think that quarks exist in isolation.

Where do the new particles come from? The best answer that I can give you is that they come from pure energy. Remember that Einstein proved that E=mc 2 , that is that mass and energy are directly proportional to each other, with the speed of light squared being the proportionality constant. So, you can make matter out of energy and the other way around. So, one particle can "turn into" another type of particle if there is enough energy to do it (and certain particle physics book-keeping rules are met). In the case of the neutron and the proton this condition is satisfied, so the reaction can take place.

• Kids/Students
• Physics
• Mass
• Subatomic Particles
• Neutrinos
• Quarks
• Energy
• Neutrons
• Electrons
• Protons
• Decay

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

Ch. Messier, Connoissance des Temps ou des Mouvements Célestes (1781), p. 227.

W. Herschel, Philos. Trans. R. Soc. London 76, 457 (1786).

J. F. W. Herschel, Philos. Trans. R. Soc. London 2, 274 (1815).

J. Dreyer, Mem. R. Astron. Soc. 49, 1 (1888).

R. Proctor, Proc. R. Soc. London 18, 169 (1869).

W. P. Fleming, Clusters and Faint Stars, Vol. 23 of Harvard College Observatory Observations, Logs, Instrument Readings, and Calculations (1904–1911).

A. S. Eddington, Mon. Not. R. Astron. Soc. 71, 43 (1910).

J. Jeans, Mon. Not. R. Astron. Soc. 74, 109 (1913).

H. Shapley, Harvard College Observ. Bull. 874, 4 (1930).

S. Chandrasekhar, Astrophys. J. 97, 255 (1943).

A. S. Eddington, Nature (London, U.K.) 106 (2653), 14 (1920).

G. Gamow, Astrophys. J. 87, 206 (1938).

M. Schwarzschild, Leaflets Astron. Soc. Pacif. 5, 400 (1949).

C. M. Bondi and H. Bondi, Mon. Not. R. Astron. Soc. 111, 397 (1951).

F. Zwicky, Publ. Astron. Soc. Pacif. 72, 365 (1960).

W. Herschel, Phil. Trans. R. Soc. London 81, 71 (1791).

J. H. Jeans, Phil. Trans. R. Soc. London, Ser. A 199, 1 (1902).

F. Hoyle, Astrophys. J. 118, 513 (1953).

A. Blaauw, Publ. Kapteyn Astron. Lab. 51, 1 (1946).

W. A. Ambarzumjan, in Stern-Assoziationen Abhandlungen aus der Sowjetischen Astronomie, Folge 1 (Otto Singer, 1951), p. 33.

F. Zwicky, Phys. Today 6, 7 (1953).

S. Perlmutter, M. Turner, and M. White, Phys. Rev. Lett. 83, 670 (1999).

A. Toomre, Astrophys. J. 139, 1217 (1964).

A. V. Tutukov, Astron. Rep. 63, 79 (2019).

S. Trujillo-Gomez, M. Reina-Campes, and J. Kruijssen, Mon. Not. R. Astron. Soc. 488, 3972 (2019).

C. Lada and E. Lada, Ann. Rev. Astron. Astrophys. 41, 57 (2003).

A. V. Tutukov, Astron. Astrophys. 70, 57 (1978).

J. Simon, Ann. Rev. Astron. Astrophys. 57, 375 (2019).

M. Krumholz and C. McKee, Mon. Not. R. Astron. Soc. 494, 624 (2020).

T. Eubanks, arXiv: 1912.12730 (2019).

P. Kroupa, in Proceedings of IAU Symposium 241, Ed. by A. Vazdekis and R. F. Peletier (Cambridge Univ. Press, Cambridge, 2007), p. 109.

M. Gieles and H. Baumgardt, Mon. Not. R. Astron. Soc 389, L1 (2008).

J. Kapteyn, Kon. Nederl. Akad. Wetensch. Proc. 14, 524 (1911).

J. Jeans, Mo. Not. R. Astron. Soc. 76, 552 (1916).

B. Lindblad, Astrophys. J. 62, 191 (1925).

S. Chandrasekhar, Principles of Stellar Dynamics (Univ. Chicago Press, Chicago, 1942).

K. F. Ogorodnikov and I. N. Latyshev, Sov. Astron. 12, 279 (1968).

O. Eggen, Astron. J. 112, 1595 (1996).

Y. Chumak and A. Rastorguev, Astron. Lett. 32, 446 (2006).

Y. Chumak and A. Rastorguev, IAU Symp. 246, 107 (2008).

R. Ibata, G. Lewis, and N. Martin, Astrophys. J. 819, 11 (2016).

S. Bose, I. Ginsburg, and A. Loeb, Astrophys. J. 859, 13 (2018).

E. Balbinot, M. Gieles, Mon. Not. R. Astron. Soc. 474, 2479 (2018).

M. Gieles, C. Charbonnel, M. G. H. Krause, et al., Mon. Not. R. Astron. Soc. 478, 2 (2018).

R. Ibata, M. Bellazzini, and K. Melhan, Nat. Astron. 3, 667 (2019).

T. de Boer, V. Belokurov, and S. Koposov, Mon. Not. R. Astron. Soc. 451, 3489 (2015).

R. Beaton, D. Martinez-Delgado, S. Majewski, et al., Astropys. J. 790, 117 (2014).

H. Morrison, M. Mario, and E. Olszewski, ASP Conf. Proc. 273, 123 (2002).

J. Yoon, K. Johnston, and D. Hogg, Astrophys. J. 731, 15 (2011).

A. P. Naik, N. W. Evans, E. Puchwein, H. Zhao, and A. C. Davis, arXiv: 2002.05738 (2020).

J. M. Diederik Kruijssen, J. L. Pfeffer, M. Chevance, A. Bonaca, et al., arXiv: 2003.01119 (2020).

A. Fattahi, A. Deason, and C. Frenc, arXiv: 2002.12043 (2020).

P. Boltrini, R. Mohayaee, and J. Silk, arXiv: 2002.12192 (2020).

T. Antoja, P. Rames, C. Mateo, et al., arXiv: 2002.10012 (2020).

R. Ibata, M. Bellazzini, G. Thomas, K. Malhan, N. Martin, B. Famaey, and A. Siebert, Astrophys. J. Lett. 891, 1 (2020).

A. Alabi, D. A. Forbes, A. J. Romanowsky, and J. P. Brodie, Mon. Not. R. Astron. Soc. 491, 5693 (2020).

V. Afanasiev, A. Moiseev, and A. Smirnova, Astrophys. Bull. 75, 12 (2020).

E. Roebler, R. Buscicchio, and A. Vecchio, arXiv:2002.10465 (2020).

E. Krugel and A. V. Tutukov, Astron. Astrophys. 275, 416 (1993).

S. Ratzenbock, S. Meingast, J. Alves, T. Möller, and I. Bomze, arXiv: 2002.05728 (2020).

A. Riley and L. Strigari, Mon. Not. R. Astron. Soc. 494, 983 (2020).

R. Ibata, K. Malhan, N. Martin, and E. Starkenburg, Astrophys. J. 865, 85 (2018).

N. Arakelyan, S. Pilipenko, and N. Libeskind, Mon. Not. R. Astron. Soc. 481, 918 (2018).

A. Duncan, Mon. Not. R. Astron. Soc. 493, 847 (2020).

B. Ratcliffe, M. Neiss, K. Johnston, and B. Sen, arXiv: 2002.07183 (2020).

M. Salaris, S. Cassisi, A. Mucciarelli, and D. Nardiello, Astron. Astrophys. 629, 6 (2019).

R. Ibata, K. Malhan, and N. Martin, Astrophys. J. 872, 23 (2019).

H. Koppelman, A. Helmi, D. Massari, A. M. Price-Whelan, and T. K. Starkenburg, Astron. Astrophys. 631, L9 (2019).

A. V. Tutukov, G. Lazareva, and I. Kulikov, Astron. Rep. 55, 770 (2011).

S. Kavirai, arXiv: 2001.01728 (2020).

J. Bovy, Astrophys. J. Suppl. Ser. 216, 2 (2015).

M. Miyamoto and R. Nagai, Publ. Astron. Soc. Jpn. 27, 533 (1975).

J. Navarro, C. Frenk, and S. White, Astrophys. J. 462, 563 (1996).

Z. M. Malkin, Astron. Rep. 57, 128 (2013).

P. J. McMillan, Mon. Not. R. Astron. Soc. 465, 1 (2017).

R. Abuter, A. Amorim, M. Bauböck, J. P. Berger, et al. (Gravity Collab.), Astron. Astrophys. 625, L10 (2019).

E. S. Postnikova, N. V. Chupina, and S. V. Vereshchagin, INASAN Sci. Rep. 3, 336 (2019).

N. Robichon, Y. Lebreton, and F. Arenou, Astrophys. Space Sci. 265, 279 (1999).

R. J. Dodd, Mon. Not. R. Astron. Soc. 355, 959 (2004).

D. Barrado y Navascués, J. R. Stauffer, and R. Jayawardhana, Astropys. J. 614, 386 (2004).

I. Platais, C. Melo, J.-C. Mermilliod, V. Kozhurina-Platais, J. P. Fulbright, R. A. Méndez, M. Altmann, and J. Sperauskas, Astron. Astrophys. 461, 509 (2007).

N. Lodieu, A. Pérez-Garrido, R. L. Smart, and R. Silvotti, Astron. Astrophys. 628, A66 (2019).

E. S. Postnikova, W. H. Elsanhoury, D. P. Sariya, N. V. Chupina, S. V. Vereshchagin, and I.-G. Jiang, Res. Astron. Astrophys. 20, 2 (2020).

Ya. O. Chumak and A. S. Rastorguev, Astron. Lett. 32, 3 (2006).

S. J. Aarseth ans J. Sverre, Gravitational N-Body Simulations (Cambridge Univ. Press, Cambridge, 2003).

N. V. Kharchenko, P. Berczik, M. I. Petrov, A. E. Piskunov, S. Röser, E. Schilbach, and R.-D. Scholz, Astron. Astrophys. 495, 3 (2009).

P. van Dokkum, C. Gilhuly, A. Bonaca, A. Merritt, et al., Astrophys. J. Lett. 883, 2 (2019).

N. Shipp, A.Drlica-Wagner, E. Balbinot, P. Ferguson, et al., Astrophys. J. 862, 114 (2018).

J. L. Carlin, C. T. Garling, A. H. G. Peter, D. Crnojević, et al., Astrophys. J. 886, 11 (2019).

## Title: The Evaporation and Survival of Cluster Galaxies’ Coronae. II. The Effectiveness of Anisotropic Thermal Conduction and Survival of Stripped Galactic Tails

We simulate anisotropic thermal conduction between the intracluster medium (ICM) and the hot coronal interstellar medium (ISM) gas in cluster galaxies. In Paper I, we simulated the evaporation of the hot ISM due to isotropic (possibly saturated) conduction between the ISM and ICM. We found that hot coronae evaporate on ∼10 Myr timescales, significantly shorter than the ∼10 Myr gas loss times due to ram pressure stripping. No tails of stripped gas are formed. This is in tension with the observed ubiquity and implied longevity of compact X-ray coronae and stripped ISM tails, and requires the suppression of evaporation, possibly due to magnetic fields and anisotropic conduction. We perform a series of wind tunnel simulations similar to that in Paper I, now including ISM and ICM magnetic fields. We simulate the effect of anisotropic conduction for a range of extreme magnetic field configurations: parallel and perpendicular to the ICM wind, and continuous and completely disjointed between the ISM and ICM. We find that when conduction is anisotropic, gas loss due to evaporation is severely reduced the overall gas loss rates with and without anisotropic conduction do not differ by more than 10%–20%. Magnetic fields also prevent stripped tailsmore » from evaporating in the ICM by shielding, and providing few pathways for heat transport between the ICM and ISM. The morphology of stripped tails and magnetic fields in the tails and wakes of galaxies are sensitive to the initial magnetic field configuration. « less

## What is the characteristic time of the evaporation of the galaxies? - Astronomy

What causes a fundamental particle (e.g. one of the heavier quarks) to "decay" into other fundamental particles, and where do these new particles come from if they are not part of the original particle?

It seems to me that for a particle to decay, it must have some sort of internal or external force acting on it, but how is this possible if all forces are borne by other fundamental particles? And as to the new particles formed, I am perplexed by their ability to spring into existence after the original particle ceases to exist. I am a high school student with some knowledge of physics, but I have only recently begun reading about particle physics.

In a sense, particles will decay because they are lazy: they want to be in the lowest possible energy state they can reach. So, if the decay products have lower energy than the initial particle, the decay can happen spontaneously. That means that the particle can be sitting in the middle of nowhere with absolutely no forces acting on it, and it will still decay. Though it is not possible to predict the exact time at which the decay will happen, the particles have a characteristic lifetime which they typically live (this is very close to their half-life, if you've heard of that term before).

As an example, a neutron is slightly heavier than a proton, so it has slightly more energy than the latter. It turns out that left alone, a free neutron (one that isn't bound in a nucleus) will spontaneously decay into a proton, and electron and a neutrino (this is called "beta decay"). The characteristic time for the decay to occur is about 15 minutes.

Finally, what kinds of particles can decay in this way? It turns out that any particles that are composites of fundamental particles (like protons, neutrons, and atoms full of protons and neutrons) can decay in this manner. As for fundamental particles themselves, an electron for example can't spontaneously change into anything else in the same way that a neutron decays. The quarks which you mention are a more difficult case, because we don't think that quarks exist in isolation.

Where do the new particles come from? The best answer that I can give you is that they come from pure energy. Remember that Einstein proved that E=mc 2 , that is that mass and energy are directly proportional to each other, with the speed of light squared being the proportionality constant. So, you can make matter out of energy and the other way around. So, one particle can "turn into" another type of particle if there is enough energy to do it (and certain particle physics book-keeping rules are met). In the case of the neutron and the proton this condition is satisfied, so the reaction can take place.

• Kids/Students
• Physics
• Mass
• Subatomic Particles
• Neutrinos
• Quarks
• Energy
• Neutrons
• Electrons
• Protons
• Decay

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

## What is the characteristic time of the evaporation of the galaxies? - Astronomy

What causes a fundamental particle (e.g. one of the heavier quarks) to "decay" into other fundamental particles, and where do these new particles come from if they are not part of the original particle?

It seems to me that for a particle to decay, it must have some sort of internal or external force acting on it, but how is this possible if all forces are borne by other fundamental particles? And as to the new particles formed, I am perplexed by their ability to spring into existence after the original particle ceases to exist. I am a high school student with some knowledge of physics, but I have only recently begun reading about particle physics.

In a sense, particles will decay because they are lazy: they want to be in the lowest possible energy state they can reach. So, if the decay products have lower energy than the initial particle, the decay can happen spontaneously. That means that the particle can be sitting in the middle of nowhere with absolutely no forces acting on it, and it will still decay. Though it is not possible to predict the exact time at which the decay will happen, the particles have a characteristic lifetime which they typically live (this is very close to their half-life, if you've heard of that term before).

As an example, a neutron is slightly heavier than a proton, so it has slightly more energy than the latter. It turns out that left alone, a free neutron (one that isn't bound in a nucleus) will spontaneously decay into a proton, and electron and a neutrino (this is called "beta decay"). The characteristic time for the decay to occur is about 15 minutes.

Finally, what kinds of particles can decay in this way? It turns out that any particles that are composites of fundamental particles (like protons, neutrons, and atoms full of protons and neutrons) can decay in this manner. As for fundamental particles themselves, an electron for example can't spontaneously change into anything else in the same way that a neutron decays. The quarks which you mention are a more difficult case, because we don't think that quarks exist in isolation.

Where do the new particles come from? The best answer that I can give you is that they come from pure energy. Remember that Einstein proved that E=mc 2 , that is that mass and energy are directly proportional to each other, with the speed of light squared being the proportionality constant. So, you can make matter out of energy and the other way around. So, one particle can "turn into" another type of particle if there is enough energy to do it (and certain particle physics book-keeping rules are met). In the case of the neutron and the proton this condition is satisfied, so the reaction can take place.

• Kids/Students
• Physics
• Mass
• Subatomic Particles
• Neutrinos
• Quarks
• Energy
• Neutrons
• Electrons
• Protons
• Decay

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

## Galaxy Lens Minimum Distance

The large size of a galaxy means that we must be very far away to see its lens. The minimum distance is given by the same equation that sets the minimum distance we must be from a star to see it's lens. If we take the radius where the density of a galaxy fall rapidly to be 3 kpc, and give it a mass of 10 12 solar masses, which are the values that characterize our own Milky Way Galaxy, we find that we see the galaxy's lens when the galaxy is more than 100 Mpc away, which is much farther than our neighboring galaxies—less than 1Mpc—but much farther than the edge of the universe—more than 4,000 Mpc.

The limit on how far a galaxy must be for its lens to be visible is proportional to R 2 /M. Most galaxies are much smaller than our own, but many of these would still have visible lenses.

## What is the characteristic time of the evaporation of the galaxies? - Astronomy

What causes a fundamental particle (e.g. one of the heavier quarks) to "decay" into other fundamental particles, and where do these new particles come from if they are not part of the original particle?

It seems to me that for a particle to decay, it must have some sort of internal or external force acting on it, but how is this possible if all forces are borne by other fundamental particles? And as to the new particles formed, I am perplexed by their ability to spring into existence after the original particle ceases to exist. I am a high school student with some knowledge of physics, but I have only recently begun reading about particle physics.

In a sense, particles will decay because they are lazy: they want to be in the lowest possible energy state they can reach. So, if the decay products have lower energy than the initial particle, the decay can happen spontaneously. That means that the particle can be sitting in the middle of nowhere with absolutely no forces acting on it, and it will still decay. Though it is not possible to predict the exact time at which the decay will happen, the particles have a characteristic lifetime which they typically live (this is very close to their half-life, if you've heard of that term before).

As an example, a neutron is slightly heavier than a proton, so it has slightly more energy than the latter. It turns out that left alone, a free neutron (one that isn't bound in a nucleus) will spontaneously decay into a proton, and electron and a neutrino (this is called "beta decay"). The characteristic time for the decay to occur is about 15 minutes.

Finally, what kinds of particles can decay in this way? It turns out that any particles that are composites of fundamental particles (like protons, neutrons, and atoms full of protons and neutrons) can decay in this manner. As for fundamental particles themselves, an electron for example can't spontaneously change into anything else in the same way that a neutron decays. The quarks which you mention are a more difficult case, because we don't think that quarks exist in isolation.

Where do the new particles come from? The best answer that I can give you is that they come from pure energy. Remember that Einstein proved that E=mc 2 , that is that mass and energy are directly proportional to each other, with the speed of light squared being the proportionality constant. So, you can make matter out of energy and the other way around. So, one particle can "turn into" another type of particle if there is enough energy to do it (and certain particle physics book-keeping rules are met). In the case of the neutron and the proton this condition is satisfied, so the reaction can take place.

• Kids/Students
• Physics
• Mass
• Subatomic Particles
• Neutrinos
• Quarks
• Energy
• Neutrons
• Electrons
• Protons
• Decay

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

## Title: Discovery of a ∼5 day characteristic timescale in the Kepler power spectrum of Zw 229–15

We present time series analyses of the full Kepler data set of Zw 229–15. This Kepler light curve—with a baseline greater than 3 yr, composed of virtually continuous, evenly sampled 30 minute measurements—is unprecedented in its quality and precision. We utilize two methods of power spectral analysis to investigate the optical variability and search for evidence of a bend frequency associated with a characteristic optical variability timescale. Each method yields similar results. The first interpolates across data gaps to use the standard Fourier periodogram. The second, using the CARMA-based time-domain modeling technique of Kelly et al., does not need evenly sampled data. Both methods find excess power at high frequencies that may be due to Kepler instrumental effects. More importantly, both also show strong bends (Δα ∼ 2) at timescales of ∼5 days, a feature similar to those seen in the X-ray power spectral densities of active galactic nuclei (AGNs) but never before in the optical. This observed ∼5 day timescale may be associated with one of several physical processes potentially responsible for the variability. A plausible association could be made with light-crossing dynamical or thermal timescales depending on the assumed value of the accretion disk size and on unobservedmore » disk parameters such as α and H/R. This timescale is not consistent with the viscous timescale, which would be years in a ∼10 M AGN such as Zw 229–15. However, there must be a second bend on long (≳ 1 yr) timescales and that feature could be associated with the viscous timescale. « less

## The expanding universe

Scott Dodelson , Fabian Schmidt , in Modern Cosmology (Second Edition) , 2021

### 2.4.3 Dark matter

As we mentioned in Ch. 1 , the overwhelming evidence for (non-baryonic) dark matter is not a new revelation to astronomers, who have found corresponding evidence within our Milky Way and local group, as well as other galaxies and clusters of galaxies. But how do we measure the total density of matter? Unlike for baryons, we cannot use nuclear and atomic physics, but have to rely on gravity.

The anisotropies in the CMB (Ch. 9 ) provide a measurement of the physical matter density parameter Ω m h 2 . The sensitivity of the CMB to the matter density is both due to the effect of matter on the expansion history in the early universe, as well as the fact that dark matter dominates the gravitational potential wells which also leave their imprint in the CMB anisotropies. Assuming the concordance model, the Planck team reported Ω m h 2 = 0.1431 ± 0.0025 ( Planck Collaboration, 2018b ). Therefore, again invoking our knowledge of the Hubble constant, the CMB observations are consistent with a matter density equal to about 30% of the critical density.

The distance-redshift relation in the late universe, as probed by standard candles and rulers, constrains Ω m alone. When combined with the CMB, the constraint becomes very tight, yielding Ω m = 0.311 ± 0.006 .

As we will see in Ch. 11 and Ch. 13 , large-scale structure provides two beautiful ways to probe gravitational potential wells and hence the amount of matter: galaxy velocities and gravitational lensing. Velocities are probed through the characteristic distortion they imprint on the three-dimensional statistics of galaxy number counts. Gravitational lensing is detected through the statistics of galaxy shapes. As an example, measurements of weak gravitational lensing and galaxy clustering using the first year of data from the Dark Energy Survey resulted in a constraint of Ω m = 0.27 − 0.02 + 0.03 ( Abbott et al., 2018 ). The slight discrepancy between this number and those driven mostly by the CMB is useful to point out (even though both may have changed slightly by the time you read this) because it (i) highlights the robust conclusion from all probes that the total matter density is roughly 30% of the critical density and ( i i ) acknowledges that at any given time, there are often hints of tension in the values of parameters inferred from different probes. Whether these are simply statistical fluctuations that will go away with more data, or indicate profound cracks in the concordance model, is one of the exciting open questions in modern cosmology.

Finally, another way of measuring the total mass density is to pick out observations sensitive to Ω b / Ω m and use the value of Ω b , determined through either BBN or CMB, to infer the matter density. Massive galaxy clusters are perhaps the most promising target, since most of the baryonic mass in a galaxy cluster is in the form of hot gas which is observable through its thermal X-ray emission or the so-called Sunyaev–Zelɽovich (SZ) effect (see Sect. 12.5 and Sect. 11.3 , respectively). If this ratio is characteristic of the universe as a whole—it probably is to a good approximation—then the cosmic baryon to matter ratio is Ω b / Ω m = ( 0.089 ± 0.012 ) h − 3 / 2 ( Mantz et al., 2014 ). Since baryons make up about 5% of the critical density, the total matter density is inferred again to be roughly 30% of the critical density.

We conclude that there is now agreement among a wide variety of probes that the total matter density in the universe is about 30% of the critical density, with 80% of that being in the form of non-baryonic dark matter.