Isotropy and Homogeneity

Transcription

1 Cosmic inventory

2 Isotropy and Homogeneity On large scales the Universe is isotropic (looks the same in all directions) and homogeneity (the same average density at all locations. This is determined from measurements of the microwave background.

3 Hydrogen and Helium Abundance The Big Bang created about 75% hydrogen and 25% helium and trace amounts of lithium and beryllium. The cosmic density of helium is slowly increasing over time from stellar nucleosynthesis, but only very marginally. The reason for the 75% and 25% is the balance between two effects, one cosmological and one physical 1) the cosmic temperature variation over time, which depends on the expansion of the Universe immediately following inflation and the total amount of energy dumped in the Universe by inflation. 2) the way the nuclear physics reaction rates very with temperature.

4 Following inflation, the Universe cools approximately as t -1/2. The nuclear reaction rates depend on temperature.

5 Milky Way abundances Almost all of the gas is H (about 75%) and He (about 25%), which was created in the Big Bang. Other elements are only present in very small amounts. The conditions where another element become the dominant one must be very specific. The light elements Li, Be, and B are destroyed in stars and so are very rare. The next most common elements after hydrogen and helium are CNO, which are produced in red giant atmospheres. Heavier elements are produced in supernovae. Iron is the stable end product of nuclear reactions in many supernovae so is very common. The heaviest elements produced naturally in supernovae are Th and U.

6 Critical Density The critical density is the density of a flat universe in the absence of dark energy. It is calculated in terms of the other cosmological parameters from the cosmological field equations. In a later lecture we will see how the cosmological field equations come from the equations of general relativity and the Friedmann Robertson Walker metric for isotropic and homogeneous space.

7 Cosmic Density of Stars The galaxy stellar mass function describes the number of galaxies as a function of their mass in stars. (Baldry et al. 2008) Summing the galaxy stellar mass function overall masses equals the cosmic density in stars (Dickinson et al. 2003)

8 Star formation in galaxies Star formation happens in two modes in galaxies. Quiescent mode, mostly in spiral galaxies Burst mode, mostly in which galaxies merge and become infrared galaxies

9 Cosmological Density of Gas in Galaxies Most of the gas in galaxies is atomic hydrogen. The amount of atomic hydrogen in each galaxy can be determined by the strength of the 21 cm HI line. A HI mass function can be determined in the same way the stellar mass function is determined. Summing the HI mass function then gives the cosmological density of gas in galaxies. (Santos et al. 2015)

10 Cosmological Density of Intergalactic Medium Simulations predict most baryons are in a warm-hot intergalactic medium. This is heated to about 10 6 K by shocks from star formation over the history of the Universe, mostly early on. This gas has extremely low density and is very difficult to detect. It can be observed because the very small number of metals present can generate absorption features in featureless spectra of distant objects. HST ultraviolet spectrum of the quasar PG (Richter et al. 2008)

11 Radiation Photons of all energies are present in the Universe. Only the cosmic microwave background is primordial. Photons at other wavelengths were generated by astrophysical objects, mostly stars. The energy density in photons is much smaller than in matter. The opposite was true immediately following inflation. The reason is that the radiation density falls a lot more than the matter density as the universe expands is because matter and radiation have a different equation of state and pressure and density are related in different ways.

12 Big Bang Nucleosynthesis h is the Hubble constant in units of 100 km/s, equal to 0.7 The cosmological baryon density is 0.03 b < 0.04.

13 Cosmological baryon density in the range 0.03 < Ω b < In stars in galaxies Ω = In atomic gas in galaxies Ω HI = Elliptical galaxies do not contain atomic gas. Instead, they contain hot ionized gas but this contains a small fraction of the stellar mass. Infrared galaxies contain mostly molecular gas for star formation and very little atomic gas. However, these galaxies are rare. Stars between galaxies are difficult to observe. The maximum number of stars that could reside in a diffuse low surface brightness component between galaxies is about 15%. The cosmological density of stellar remnants like neutron stars and black holes is small. Otherwise the abundance of heavy elements would be overproduced. Most baryons in the Universe are thought to reside in the warm-hot intergalactic medium (WHIM). This can be inferred from cosmological simulations as well. In these simulations, rich clusters of galaxies, which contain large quantities of x-ray gas (about three times as much by mass as the stars in the galaxies), can be thought of as accumulations of the WHIM in regions of much higher than average density.

14 X-ray gas in galaxy clusters Galaxy clusters are surrounded by large amounts of hot ionized gas, typically three times the combined mass of all the stars in the galaxies. This hot gas can be observed from its X-ray emission. In cosmological simulations, clusters of galaxies are seen at the intersection of filaments. The cluster x-ray halos then represent accumulations of the WHIM at these intersections.

15 Dark Matter Most of the mass in galaxies is in dark matter. This can be inferred from rotation curves, dynamics, and gravitational lensing. The relative amounts of dark matter and baryonic matter can be determined from measurements of galaxy clusters. This is because detailed mass profiles can be determined from x-ray observations and gravitational lensing. The assumption is that clusters are large enough to represent the cosmic average. (G.Smith, University of Birmingham) The data fit well with a Cold Dark Matter model with DM = 0.3 Galaxy clusters have the ratio by mass dark matter: x-ray gas: galaxies 70:8:1 Bulk motions of galaxies on large scales also imply DM = 0.3

16 Cold Dark Matter The dark matter is thought to be cold because otherwise the dark matter perturbations would diffuse away to nothing and galaxies would never form. CDM models reproduce most aspects of the galaxy population very well. The predicted Galaxy correlation function matches the observations to a high degree of accuracy. CDM theory has potential difficulties in reproducing the detailed structure of individual galaxies.

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19 Dark energy Dark energy is repulsive, meaning that mass pushes other mass away, like antigravity. This leads to an unusual equation of state. The cosmological constant is one form of dark energy with w=-1. This corresponds to the false energy of the vacuum. The vacuum a zero point energy because of processes like electro-positron creation (as far as we can tell, because we have not probed sizes below the Planck scale). Calculations of the cosmological constant over the regime larger than the Planck scale gives a value of the cosmological constant too large by 123 orders of magnitude.

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21 Precision cosmology Cosmological densities are expressed in units of the critical density

22 Cosmic inventory

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24 Future supernova 1a surveys Blue = optical Red = near-infrared Filled = existing Shaded = future

25 HI cosmology surveys