Open Course : Astronomy : Introduction : Lecture 36 : Evolution of the Universe

36.1 The Big Bang

If the Universe is expanding, at some time it must have been concentrated in a single point, with infinite density.

This point is called the cosmic singularity.

This singularity is like that of a black hole, in that all matter and energy were crushed together at a single point.

Some sort of "explosion" must have occurred to start the expansion of the Universe outward from the cosmic singularity; it is called the Big Bang.

This is not like an explosion in which debris goes flying off into space.

Instead, it is the expansion of space itself, and therefore occurred everywhere simultaneously.

The Hubble Constant

H₀ ~ 21 Km/s/Mly = 68 Km/s/Mpc

gives a rough estimate of the time since the Big Bang (i.e. the age of the Universe):

time = distance/velocity = r/v = 1/H₀ = 13 Gy.

Other estimates put it at as little as 8 Gy, or as much 16 Gy.

In any case this must certainly be an overestimate, because the gravitational pull of the Universe's mass has slowed down the expansion (i.e. H₀ has decreased with time).

This is a problem, because stellar theory indicates that the oldest stars are at least 14 Gy old.

There has therefore been a great deal of controversy about the exact value of the Hubble Constant.
Any galaxies which are farther than 13 Gly away we cannot see, because there hasn't been enough time for the light to arrive here.

The spherical surface at distance of 13 Gly from us is called the cosmic particle horizon; the observable universe lies inside this surface.

36.2 The Cosmic Microwave Background

There is additional evidence for the Big Bang besides the observed expansion of the Universe.

Because all of the observed mass and energy in the Universe must have been initially concentrated in a very small space, the mass/energy density must have been very high, which means a very high temperature, hotter than the interior of any star.

At these temperatures, most of the mass/energy existed in the form of high-energy gamma radiation. These photons interacted with each other and with other particles, gaining and losing energy as they collided, resulting in a variety of wavelengths with a blackbody distribution.

As the Universe expanded in size, the mass/energy density decreased and the temperature dropped. The blackbody spectrum must have therefore shifted its peak to longer wavelengths as well, as described by Wien's law:

Now, 13 Gy later, the photons have shifted to low-energy microwaves.

This radiation is known as the cosmic microwave background. Its existence is not predicted by any other theory of the Universe.

It was discovered by Penzias and Wilson at Bell Labs in the early 1960s; they were working on a microwave antenna to relay telephone calls to communications satellites, and found a background noise with a peak wavelength around 1 mm, corresponding to T = 3 K.

The Cosmic Background Explorer (COBE) satellite was sent into orbit in 1989 to measure the background radiation; it was found to be a perfect blackbody spectrum for T = 2.735 K.

COBE also found that the background is almost perfectly isotropic.

There do exists slight anisotropies, however . For one thing, the radiation is slightly "warmer" (blueshifted) in the direction of the constellation Leo, and "cooler" (redshifted) towards Aquarius. The following image is a projection of the entire sky, with the Milky Way horizontal across the middle and Sagittarius in the center. (Note: the colors are actually the reverse of what might be expected; Leo is in the red region at the upper right and Aquarius is in the blue region at the lower left.)

This smooth variation in the background radiation is due to the motion of the Earth with respect to the background.

In the direction we are travelling a blueshift occurs (the same as if we were standing still and it moved towards us).

Analysis of the data indicates that we are moving at a speed of 390 km/s towards Leo.

Taking into account our motion around the galaxy (horizontally to the right), this means that the entire galaxy must be moving at a speed of 600 km/s in the direction of Centaurus, somewhat closer to the center of this figure (in the green).

We are pulled in that direction by the gravitational force of several nearby galaxy clusters (including the Virgo Cluster), and a gigantic supercluster called the Great Attractor.

When the motion of the Earth is accounted for, remaining fluctuations in the background radiation are still found, although they are at most 100 µK warmer or cooler than the average:

These variations are believed to be due to concentrations of mass in the early universe, which prevented complete isotropy of the background radiation by gravitationally redshifting it (regions of greater density appear blue here).

This mass eventually condensed into the superclusters, clusters, and galaxies we now observe.

36.3 The First Few Instants

No one knows what caused the Big Bang initially, but once it occurred, we know that the Universe underwent many changes as it expanded and its temperature decreased. The first few instants, in particular, resulted in a rapid set of developments.

During an initial short period of time called the Planck time = 10^-43 s, mass and energy were so concentrated that space and time were not describable by our current knowledge of physics (much like in the immediate vicinity of a black hole's singularity).

All of the forces of nature were unified, with no distinction between how they affected particles. This is quite different from the present case, where, for example, the electric repulsion between a pair of electrons is 10⁴² times larger than their gravitational attraction.

After the Planck time, the temperature had decreased to 10³² K, which allowed gravity to separate out from the other forces of nature to become its own distinct, weaker interaction.

Although the remaining forces continued in their unified condition, this high-energy state is familiar to physicists from their studies of the sub-atomic world using particle accelerators.

36.4 The Creation of Matter

Most of the mass of the Universe was created throughout the first second of its existence, via a process called pair production. At these high energies, pairs of photons can collide and produce particle/antiparticle pairs such as electrons and positrons:

Pair production can result in many different types of particles, but only so long as the photons have at least as much energy as the total mass of the particles. So, as the temperature of the Universe decreased and photons had less energy, less and less massive particles could be produced.

Pair production of all types ended by a time of about 1 s, when the temperature had dropped to 6 x 10⁹ K.

Pair production is the reverse of the pair annihilation that occurs in the core of the Sun, where electrons and positrons collide to produce photons. As more and more particles were created, pair annihilation also increased.

Once a particular type of particle could no longer be produced, it rapidly disappeared because pair annihilation can always occur, independent of temperature.

Due to a symmetry breaking in the pair production process, slightly more matter than antimatter was produced, maybe one particle in a billion. Therefore, once all of the antimatter was destroyed, only a small amount of matter remained, which is the mass we see today.

36.5 The Formation of Nuclei

Recall that the elemental abundance of stars is about 74% H, 25% He, 1% others.

The 1% other is known to be produced inside of stars themselves, but only 10% of the helium can be understood as being produced inside of stars.

Where did the rest of the He come from?

The Big Bang could explain this, however; immediately afterward, it would have been so hot that a large amount of H fusion would occur everywhere in space.

After about 300,000 y the temperature in the Universe decreased to about 3000 K, corresponding to a peak wavelength in the near infrared. Before this time, the Universe was very much like the interior of a star: all matter existed as a plasma of charged particles, because it was too hot for neutral atoms to exist, and the Universe was opaque, because photons couldn't travel very far before being scattered.

After this time, however, matter was largely converted into neutral atoms. This decoupling happened in a matter of seconds, letting photons travel relatively freely through the Universe.

Open Course Info

Introduction to Astronomy

Lecture 36: The Evolution of the Universe

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.

-- Douglas Adams, Hitchhiker's Guide to the Galaxy

36.1 The Big Bang

36.2 The Cosmic Microwave Background

36.3 The First Few Instants

36.4 The Creation of Matter

36.5 The Formation of Nuclei