The History of the Universe

Universe expands as time passes

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coo

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The Early History of the UniverseElectron

Positron

Gamma-ray photon

Electrons, positrons, and gamma-rays in equilibrium between pair

production and annihilation

For reasons not completely understood, there was a very slight excess of ordinary matter over antimatter (by about 1 part in 109). This is why there was still some ordinary matter left over when all the antimatter had been annihilated. (This must be the case, otherwise you wouldn't be here!) All of the protons, neutrons, and electrons in matter today were created in the first few seconds after the Big Bang.

The Early History of the Universe (2)

Protons and neutrons form a few helium nuclei; the rest of protons

remain as hydrogen nuclei

Almost no elements heavier

than helium are produced.

25% of mass in helium 75% in hydrogen

No stable nuclei with 5 and 8 protons

Cosmic Abundance of Helium and HydrogenThe Big Bang theory provides a natural way to explain the present abundance of the elements. At about 2 to 3 minutes after the Big Bang, the expanding universe had cooled to below about 109 K so that protons and neutrons could fuse to make stable deuterium nuclei (a hydrogen isotope with one proton and one neutron) that would not be torn apart by energetic photons. Protons react to produce deuterium, deuterium nuclei react to make Helium-3 nuclei, and Helium-3 nuclei react to make the stable Helium-4 nucleus.

The deuterium nucleus is the weak link of the chain process, so the fusion chain reactions could not take place until the universe had cooled enough. The exact temperature depends sensitively on the density of the protons and neutrons at that time. Extremely small amounts of Lithium-7 were also produced during the early universe nucleosynthesis process. After about 15 minutes from the Big Bang, the universe had expanded and cooled so much that fusion was no longer possible. The composition of the universe was 10% helium and 90% hydrogen (or if you use the proportions by mass, then the proportions are 25% helium and 75% hydrogen). Except for the extremely small amounts of the Lithium-7 produced in the early universe, the elements heavier than helium were produced in the cores of stars.

Fig. 15-8, p.303

Fig. 15-14, p.311

The Nature of Dark MatterCan dark matter be composed of normal matter?

• If so, then its mass would mostly come from protons and neutrons = baryons

• The density of baryons right after the big bang leaves a unique imprint in the abundances of deuterium and lithium.

• Density of baryonic matter is only ~ 4 % of critical density.

• Most dark matter must be non-baryonic!

The Early History of the Universe (3)

Photons are incessantly scattered by free electrons; photons are in

equilibrium with matter

Radiation dominated era

Photons have a blackbody spectrum at the same temperature

as matter.

RecombinationProtons and electrons recombine

to form atoms => universe becomes transparent for photons

Transition to matter dominated era

z ≈1000

The Cosmic Background Radiation (2)After recombination, photons can travel freely through space.

Their wavelength is only stretched (red shifted) by cosmic expansion.

Recombination:

z = 1000; T = 3000 K

This is what we can observe today as the cosmic background radiation!

ReionizationAfter less than ~ 1 billion years, the first stars form.

Formation of the first stars

Ultraviolet radiation from the first stars re-ionizes gas in the early universe

Reionization

universe becomes

opaque again

Large Scale Structure (2)

A large survey of distant galaxies

shows the largest structures in the

universe:

Filaments and walls of galaxy

superclusters, and

voids, basically empty space.

Cosmology with the Cosmic Microwave Background

If the universe were perfectly homogeneous on all scales at the time of recombination (z = 1000), then the CMB should be perfectly isotropic over the sky.

Instead, it shows small-scale fluctuations:

The universe could not have been perfectly uniform, though. The universe must have been slightly lumpy to form galaxies later on from the internal gravity of the lumps. Initial density variations had to exist in order to provide some direction to where surrounding matter could be attracted. The COBE satellite found slight variations in the brightness of the background radiation of about 1 part in 100,000. The slight variations exist because some parts of the universe were slightly denser than other parts. The slightly denser regions had more gravity and attracted more material to them while the expansion occurred. Over time, the denser regions got even denser and eventually formed galaxies about 1 billion years after the Big Bang.

Fluctuations of the CMB temperature

Evidence for the formation of galaxies and large-scale structure

The cosmic microwave background radiation can be explained only by the Big Bang theory. The background radiation is the relic of an early hot universe. The Big Bang theory's major competitor, called the Steady State theory, could not explain the background radiation, and so fell into disfavor.

The amount of activity (active galaxies, quasars, collisions) was greater in the past than now. This shows that the universe does evolve (change) with time. The Steady State theory says that the universe should remain the same with time, so once again, it does not work.

The number of quasars drops off for very large redshifts (redshifts greater than about 50% of the speed of light). The Hubble Law says that these are for large look-back times. This observation is taken to mean that the universe was not old enough to produce quasars at those large redshifts. The universe did have a beginning.

The abundance of hydrogen, helium, deuterium, lithium agrees with that predicted by the Big Bang theory. The abundances are checked from the spectra of the the oldest stars and gas clouds which are made from unprocessed, primitive material. They have the predicted relative abundances.

Observations are consistent with Hot Big Bang Model

Fate of the Universe Depends on mass-energy density (Curvature of Space)

The more mass there is, the more gravity there is to slow down the expansion. Is there enough gravity to halt the expansion and recollapse the universe or not? If there is enough matter (gravity) to recollapse the universe, the universe is ``closed''. In the examples of curved space above, a closed universe would be shaped like a four-dimensional sphere (finite, but unbounded). Space curves back on itself and time has a beginning and an end. If there is not enough matter, the universe will keep expanding forever. Such a universe is ``open''. In the examples of curved space, an open universe would be shaped like a four-dimensional saddle (infinite and unbounded). Space curves away from itself and time has no end.

Deceleration of the Universe

• Fate of the universe depends on the matter density in the universe.

• Expansion of the universe should be slowed down by mutual gravitational attraction of the galaxies.

• Define “critical density”, c, which is just enough to slow the cosmic expansion to a halt at infinity.

Model Universes

Siz

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cale

of t

he

Un

iver

se

Time

< c => universe will expand forever

> c => Universe will collapse back

If the density of matter equaled the critical density, then the curvature of space-time by the matter would be just sufficient to make the geometry of the universe flat!

= c => Flat UniverseMaximum age of the universe:

~ 1/H0

Deriving geometry of the universe from density measurements

Orbital speeds of stars in galaxies

Faint gas shells around ellipticalsEllipticals have faint gas shells that need massive ``dark'' haloes to contain them. The gas particles are moving too quickly (they are too hot) for the gravity of the visible matter to hang onto it.

Motion of galaxies in a clusterGalaxy cluster members are moving too fast to be gravitationally bound unless there is unseen mass.

Hot gas in clustersThe existence of HOT (i.e., fast moving) gas in galaxy clusters. To keep the gas bound to the cluster, there needs to be extra unseen mass.

Quasar spectraAbsorption lines from hydrogen in quasar spectra tells us that there is a lot of material between us and the quasars. Gravitational LensingGravitational lensing of the light from distant galaxies and quasars by closer galaxies or galaxy clusters enables us to calculate the amount of mass in the closer galaxy or galaxy cluster from the amount of bending of the light. The derived mass is greater than the amount of mass in the visible matter.

Current tallies of the total mass of the universe (visible and dark matter) indicate that all matter constitutes only 27% of the critical density.

Deriving geometry of the universe from microwave background radiation

Deriving geometry of the universe from microwave background radiation

BOOMERANG Data

Fig. 15-21c, p.319

The case of a missing Universe

Dark matter accounts for only 27% of the total mass-energy density: DM = 0.27

Observations suggest that the universe is flat: = 1

The rest 70% is something else!!

Visible matter accounts for ~ 4% of the total mass-energy density: v = 0.04

The accelerating Universe

dt

tdR

tRtH

)(

)(

1)(

redshift z

dist

ance

Fig. 15-17a, p.315

Fig. 15-17b, p.315

Supernovae are too faint

Equation of state: relation between pressure P and energy density c2

= 0 for dust (no pressure) = 1/3 for radiation (very hard pressure)

Or: acceleration = )31(3

42

2

R

G

dt

Rd

Einstein’s equations:

const)1(3 R

2

2

8

3

GR

kcc

2cP

= -1 ??

To have acceleration, 02

2

dt

Rdwe must have negative pressure!

Accelerating now, but decelerating in the past?!

Fig. 15-18, p.316

Problems with standard model. Inflation

• Flatness problem

• Horizon problem

• Initial fluctuations

• Absence of magnetic monopoles

• “Fine tuning”

Solution of the Problems of the Big Bang by InflationIf this inflationary epoch really took place, it could cure all the problems of the big bang:

The tremendous expansion means that regions that we see widely separated in the sky now at the horizon were much closer together before inflation and thus could have been in contact by light signals.

The tremendous expansion greatly dilutes any initial curvature. In fact, the inflationary theory predicts unequivocally that the Universe should globally be exactly flat, and therefore that the average density of the Universe should be exactly equal to the closure density.

The rapid expansion of the Universe tremendously dilutes the concentration of any magnetic monopoles that are produced. Simple calculations indicate that they become so rare in any given volume of space that we would be very unlikely to ever encounter one in an experiment designed to search for them.

Density Fluctuations as Seeds for Galaxy Formation

Detailed considerations indicate that inflation is capable of producing small density fluctuations that can later in the history of the Universe provide the seeds to cause matter to begin to clump together to form the galaxies and other observed structure.

Fig. 15-16, p.313

What could be the reason for inflation?