Cosmology 101 Francis P.Wilkin Union College Schenectady, NY wilkinf@union

Cosmology 101

Francis P.WilkinUnion College

Schenectady, [email protected]

Outline of Presentation• Size and distance scales

• Expansion of the Universe and Hubble’s Law

• Big Bang and its relics, CMB and light element abundances

• Uniformity of CMB and inflation

• Measuring mass, dark matter, density of the Universe

• Models of the Universe (scale factor, curvature, accel.)

• High redshift supernovae, COBE and WMAP results, dark energy, dark matter, ordinary matter contributions

• Big outstanding problems!

Distance scales:Parallax gave the first measurement to

another star by Bessell 1838

Distance Scales, cont.Method of standard candles

• Cepheid variable => external galaxies (Hubble 1925-6)

• Type Ia supernovae are standard candles used over the largest distances

Size scales Galaxy 30 kly Galaxy Cluster 6-30 Mly

Supercluster up to 100 Mly

Observable Universe R ~ c T ~ 14 Gly

Sombrero Galaxy

Spitzer Space Tel

Infrared

Makeup of the U

• Matter and Energy• Matter… baryonic (protons, neutrons,

electrons, mesons, etc) and non/baryonic (unknown nature)

• Energy…light (photons), heat• Relation: E equals M c2• Spacetime: 4 dimensions known, influenced

by matter and energy through Einstein’s theory of General Relativity

Large Scale Structure

0.5

1.0

1.5

Billion Light Years

Anglo-Australian Radio Observatory

Expansion of the Universe• V. Slipher: excess of redshifted compared to blueshifted galaxies; concluded recessional motion.

• Eddington 1923 suggested this as evidence for expansion of the U, but it wasn’t yet known what galaxies were.

• Hubble 1929 galaxy redshift proportional to distance! His law of expansion:

V = H d,

where speed V is in km/s, distance d is in Mpc, H = 73 km/s/Mpc

For years the value of H was fraught with controvery until Hubble Space Telescope determined this modern value.

Hubble also discovered that the Universe is homogeneous and isotropic on large scales.

Hubble Space Telescope

The Big Bang

The expanding universe has decreasing density and temperature. Reverse extrapolation: it once had much higher density and temperature.

Hubble’s law suggests an infinite density at a specific time in the past:

Compare v = H d to speed = distance / time

If H were constant, the age of the U is 1/H of the order 10 billion years. This time of the Universe and its immediate consequences are termed the Big Bang.

Earliest possible time we can describe: Planck time 10-43 s Prior to which a quantum theory of gravity is needed. Conditions were roughly:

tp= 10-43 s, lp=10-33 cm, Tp=3.6x1032 K, ρp=1091 g/cm3

Evidence for the Big Bang• Alpher and Herman predicted a relic of the BB as a radiation field

• Found by Penzias & Wilson. Temp of 2.73 K and very uniform (Nobel Prize 1978)

• Almost a perfect blackbody (thermal) spectrum peaking in the microwave region (Nobel Prize 2006)

• A relic of the last time the U was opaque, roughly 104 K, and since then redshifted by a large factor due to the expansion of space.

• Fluctuations in the temperature of the CMB are found at a level about 10-5 and trace the seeds of future structure formation of superclusters of galaxies. (Nobel Prize 2006)

• Conditions permitted the fusion of the lightest elements (D, He, Li, Be, B) and their abundances are a fossil remnant that diagnoses early conditions. Big bang nucleosynthesis models are in good accord with measured abundances in the oldest stars.

• Ages of all stars and galaxies are less than the time predicted for the BB.

• No satisfactory explanation for the cause of the Big Bang.

Cosmic Background Explorer COBE

A quandary emerges

Conditions in the CMB are uniform for locations (two opposite directions) that would never have been in causal contact!

Only accepted explanation is expansion of the U in extremely rapid fashion “inflation” by a huge factor 1030. Matter may in fact have been largely created during this epoch which ended about 10-35 s.

Big Bang Nucleosynthesis

Relative abundances of light elements sensitive to early conditions

Wilkinson Microwave Anisotropy

Probe - WMAP

WMAP produced unprecedented images of the earliest light in the universe – the Cosmic Microwave Background (CMB)

To measure mass, we use Newton’s law of gravity

• Circular orbit speed v = √(GM/R) so M = R v2/G

• When applied to spiral galaxies, we observe increasing mass well beyond the visible stars and gas. This dark matter dominates the masses of galaxies, often by 10 to 1.

• Similar methods use random velocities for elliptical galaxies as well as galaxy clusters.

• In clusters, dark matter dominates by a larger factor

• Even with all dark matter, density of the U in known matter insufficient to close the U.

• Mass can also be studied with gravitational lensing

Dark Matter in Galaxies

Lack of declining rotation speed implies additional mass beyond stars and gas

Models of the Universe

Use Einstein’s equations of general relativity, together with homogeneity and isotropy.

Three possible curvatures (negative, zero, of positive), related to the matter-energy density of the Universe and its evolution. Comoving coordinates and a scale factor R.

Equations give the evolution of R in terms of the matter and energy content of the U, including a possible cosmological constant or vacuum energy term

TimeLine of the Universe

Acceleration and Dark Energy

• Observations of distant supernovae point to an acceleration of the U. The U was expanding more slowly in the past!

• Interpreted as due to a repulsive gravity or dark energy

associated with the vacuum.

• WMAP and COBE, combined with the supernovae evidence, yield surprisingly precise values of cosmological parameters:

Ω = 1, ΩBaryon = 0.04, ΩDM = 0.23, ΩDE=0.73

We have no clear idea of what 96% of the Universe is by mass!!

Unsolved problems

• What is the nature of dark matter?• What is the nature of dark energy?• Why are we here? (Matter-Antimatter asymmetry)• What drove the inflation of the early Universe?• What is the nature of gravity? (Additional

dimensions?)• Why do the physical parameters have their values?• What is the nature of space and time? Do they have

fundamental building blocks?