Cosmology

Olbers’s Paradox

The Universe may be infinite – if it is, why is the night sky dark?

Expansion of the Universe

Galaxies in all directions are receding from us with ever increasing speed as their distance from us increases…

Expansion of the Universe

This expansion makes it seem like we’re at the center of the Universe. But an astronomer in another galaxy (raisin)

would observe the same phenomenon.

The recessional velocities of galaxies are not Doppler shifts – space itself is expanding, carrying the galaxies with it!

Space expands as light travels from one galaxy to another, causing the wavelength of the light to increase (i.e., redden).The amount that the light is redshifted is proportional to the amount of time it had to travel – or how far it had to go.

Expansion of the Universe

“Look-back time”

As we look at galaxies with higher and higher redshifts, we are looking farther and farther back in time because of the finite speed of light.

The Cosmic Microwave Background Radiation

From beyond the most distant galaxies, we can see the remnant radiation from the Big Bang, called the Cosmic Microwave Background radiation(CMB).

We see the CMB in any direction we look.

The hot gas responsible for the CMB is very distant, so the CMB has a very large redshift of ~ 1000.

The CMB radiation was discovered in the 1960’s by Penzias and Wilson. Its spectrum closely follows a blackbody curve of temperature T = 2.73 K.

The Cosmic Microwave Background Radiation

Just after the Big Bang the Universe was incredibly hot, but it has cooled as the Universe has expanded. As the Universe cooled, matter (and then galaxies) could form.

History of the Big Bang

Primordial Nucleosynthesis

During the first few minutes the temperature and density were high enough for the creation of deuterium (2H), helium (3He, 4He), and a small amount of lithium (Li).

But that’s it! There are no stable nuclei with atomic weights of 5 or 8, so the process halted.

The amount of matter in the Universe determines its ultimate fate.

• An “open” Universe with less than the critical density of matter will continue to expand forever.

• A “closed” Universe in which the critical density is exceeded will eventually collapse in a “big crunch.”

• A “flat” Universe with just the right critical density will stop expanding after an infinite amount of time.

Dark Matter and the Critical Density

Clusters of galaxies act like gravitational “lenses” for galaxies that lie behind them. The lensing suggests the total amount of matter in clusters is > 10 times the amount of luminous matter observed. With the dark matter, the denisty of the Universe could equal its critical density.

Recall: supernova explosions are very bright. The maximum brightness of a Type Ia supernova should always be about the same.

So, Type Ia SNe are good “standard candles” for indicating distance.

An Accelerating Universe?

Type Ia Sne aredetected in very distant galaxies.

But they appear a little too faint for the distance expected from their host-galaxy redshifts.

Implication: they are farther away than expected.

If so, the expansion of the universe is accelerating!