main issue: How did matter evolve before there were
any galaxies and stars?
The Early Universe
What Do We
- Evidence from when the universe was
young: Distant objects like quasars, seen like they were
when the universe was a few billion years old; Old globular clusters,
matter and radiation that has been around since then.
- Theory and simulations: With
varying amounts of detail, we have developed the hot big bang
model, describing the universe back to perhaps 10-43
s of age! It makes some surprisingly precise predictions.
The Formation of Spacetime and Matter
- The "instant" of the big
bang: We don't know if the universe was just a point –
probably not – but the it started approximately 13.7 Gyr ago; For the first
10–43 s ["Planck time"], we don't
know how to describe the universe, because there were rapid fluctuations in
energy and warping of space and time, and the fabric of spacetime itself
had not been "woven together" yet.
- The GUT era: Three of the four
forces were unified ("grand unification"), and it lasted until
T = 1027 K, at t
= 10–35 s, when the strong nuclear
force broke away.
- Inflation: Energy liberated
at the end of the GUT era made the universe briefly expand and
flatten very fast; Small fluctuations that were present become
seeds for galaxies (and their imprints are still seen as fluctuations
in the microwave background); Would explain why the universe
is so flat, and why it looks almost the same everywhere.
- The electroweak era: Lasted
until all forces became different, like they are today [at T
= 1015 K, 10–10
- The particle era: In addition
to photons, particles like quarks, electrons, and neutrinos were
constantly being created and annihilated (quark-gluon plasma);
lasted until quarks were combined into protons and neutrons [at
T = 1012 K, 1 ms], just
slightly more abundant than antiprotons and antineutrons.
The Formation of Nuclei
- The era of nucleosynthesis: Protons
and neutrons form nuclei which then break apart, until at t
= 3 min, T = 109 K and
nuclei are stable.
- Helium production: He nuclei
are produced by a chain of reactions.
- Result/prediction: In terms
of mass, 75% of the nuclei are H, 24% He, traces of heavier ones
[there were 7 p's for each n available].
- Current observations: We now
have 25% He, generations of stars having produced additional
He nuclei; numbers agree with prediction.
- The era of nuclei: Matter is
a hot plasma of H and He nuclei, mostly; lasts 300,000 yr, until
T = 3000 K and protons and electrons stick together.
- Decoupling: The era of nuclei
ends at about 380,000 yr, when neutral atoms form and most of
the matter does not affect photons anymore; matter becomes transparent
to visible light.
- Result/prediction: The light
that was present at the time is still around, although much redshifted,
as microwaves. More energetic photons, like UV, would be absorbed
by atoms, but there were not many of those.
- Do we have evidence? The 3 K
cosmic microwave background, discovered in 1964 by Penzias and
Wilson. The radiation is the same in all directions, except for
a blue/redshift in our direction of motion, and very small fluctuations.
- Structure formation: Matter
can now form the first gas clouds, that will eventually lead
* Like water drops on a spider web, sliding toward
the nodes of the web.
The Formation of Stars
- The era of galaxies: Star formation
started after about 200 millions years, at a much higher rate
than the present one; Before then, the universe had been dark
for some time; After this time, and over a few billion years, stars grouped
into galaxies and galaxy clusters around filaments of dark matter under
the effect of gravity;* We expect to understand better these processes
with future spacecraft missions.
- Protogalaxies: With lensing, we have
started seeing galaxy "building
blocks"; Many of the early galaxies are seen as quasars, which
later evolve into regular galaxies, or sometimes still active ones.
- First star generations: It seems
that the lower abundance of heavy elements favored the formation
of large stars, and it definitely prevented stars from having
terrestrial planets around them.
page by luca bombelli <bombelli at olemiss.edu>,
modified 3 jul 2013