Lecture 28:Lecture 28:

InflationInflation

Astronomy 1143 – Spring 2014

History of Inflation

Key Ideas

Universe expanded very quickly in the early times thanks to inflation

Energy released from phase transition?

Inflation can explain

• The flatness problem

• The monopole problem

• The horizon problem

• The growth of quantum fluctuations

Our lumpy Universe comes from these tiny fluctuations, thanks to gravity

Detected of ripples in the CMB from gravity waves – predicted by inflation models

Inflation in the Early UniverseInflationInflation = a brief period of highly accelerated

expansion, early in the history of the universe.

Space expanded much faster than the speed of light

At t ≈ 10-34 seconds, the universe started expanding exponentially, doubling in size every 10-34 seconds.

Inflation ended at t ≈ 10-32 seconds, after expansion by a factor 1030.

Beginning of Inflation

What caused inflation to start?

According to the particle physicists: universe underwent a phase transition at t ≈ 10-35 s.

Phase transition associated with the end of the GUT era – separation of strong from electroweak force

The energy released by the phase transition at t ≈ 10-35 s acts (temporarily) like dark dark energyenergy.

Example of Phase Transition

The freezing of water is an example of a phase transition

Liquid water has no preferred orientation; ice does

This is called symmetry breaking

Idea is that the separation of the strong force from the electroweak force broke symmetries found in GUTs.

This releases energy.

Liquid to Ice Transition

When water goes from liquid to solid, it goes from a random state to an ordered state.

Energy is released.

During a freeze in Florida, orange trees are sprayed with water.

Why? The energy released by freezing water warms the leaves & fruit.

Inflation to the RescueIf Inflation happened it would explain• the flatness problem• the monopole problem• the horizon problem• how quantum fluctuations became the seeds of

large-scale structure

The Flatness Problem

Why should the average density of the universe (ρ) be so close to the theoretical critical density (ρcrit)?

There’s no law of nature that says ΩΩ (= ρ/ρcrit) must be equal to one.

Why not Ω = 0.01 or Ω = 100?

Since the universe is fairlyfairly close to flat today, it must have been insanelyinsanely close to flat in its early history.

Really, Really Flat

Inflation greatly increases the radius of curvature of the Universe.

How does inflation solve the flatness problem?

Suppose the radius of the universe was only one nanometer (10-9 meter) before inflation.

After inflation, the radius would be 30,000 parsecs; today, 3 trillion trillion megaparsecs.

Really, really increases the radius

The Monopole Problem

One of the frontiers of modern physics is to create the correct GUTs and TOEs

A prediction of the best models at the present time is the existence of magnetic monopoles

• Just a North or South pole by itself• Not yet detected in the laboratory

Inflation solves this by diluting the density of magnetic monopoles

After inflation, just 1 magnetic monopole every 1x1061 Mpc3, much bigger than our current horizon

The Horizon Problem

The Universe is remarkably homogeneous and isotropic

This is easy to do if the various parts of the observed Universe can share energy, etc. so that the temperature becomes uniform

But without inflation, the whole observable Universe was never in contact

400,000 ly = 1 degree on the sky not 180

Nothing can travel faster than light!

The observed Universe was in contact thanks to Inflation

But, wait! Universe isn’t homogeneous today

Inflation can help with that, too!Inflation

Inflation

Inflation

On subatomic scales, the universe is full of quantum fluctuations.

A vacuum looks empty, but it’s full of particles & antiparticles being created & destroyed.

Inflating Quantum Fluctuations

Ordinarily, these quantum fluctuations are on tiny scales.However, inflation increased tiny scales (1 nanometer) to galaxy-sized scales (30,000 parsecs)!

Regions with higher density will start to grow by gravity. We can see these regions when the Universe is 400,000 years old

Seeds of Structure

A region that was slightlyslightly denser than average will eventually become muchmuch denser than average; it’s compressed by its own gravity

Low-amplitude density fluctuations at t ≈ 400,000 years give rise to high-amplitude fluctuations at t ≈ 13.7 billion years.

Let’s look for density fluctuations in the Cosmic Microwave Background Radiation

Density fluctuations will appear as temperature fluctuations because compressed gas heats up.

Spherical Earth can be projected onto a flat map:

So can the celestial sphere:

(visible light)

Mapping the Sky

Observation: Temperature of CMB is nearly isotropicisotropic (the same in all directions).

Interpretation: early universe was nearly homogeneous homogeneous (the same in all locations).

T = 2.725 K

Mapping the CMB (color = temperature)

Observation: Temperature of CMB is slightly hotterhotter toward Leo, coolercooler toward Aquarius (on opposite side of sky).

cooler cooler →→

← ← hotterhotter

Temperature fluctuation = 1 part per 1000.

mK = 0.001 Kelvin

News Flash: The Earth is Moving

Interpretation: difference in temperature results from a Doppler shiftDoppler shift.

• Earth orbits Sun • (v ≈ 29 km/s)

• Sun orbits center of the Galaxy • (v ≈ 220 km/s)

• Galaxy falls toward Andromeda Galaxy • (v ≈ 50 km/s)

• Local Group falls toward Virgo Cluster • (v ≈ 200 km/s)

Cosmic light from direction of Leo is slightly blueshiftedblueshifted (shorter wavelength, higher temperature).

blueshifted (Leo)

redshifted (Aquarius)

Net motion: toward Leo, with a speed v ≈ 300 km/s ≈ 0.001 c.

Observation: After subtracting the effect of our motion through space, CMB still shows hot & cold spots, about 1 degree across.

Temperature fluctuation = 1 part per 100,000

← ← hotterhottercooler cooler →→

Temperature Tells Us About DensityHigher temperatures come from compressed

gases

Regions that were compressed had higher densitydensity

HotHot spots in the CMB are higher in temperature than coldcold spots by 1 part per 100,000.

Implication: the densitydensity fluctuationsfluctuations in the early universe were also small (about 1 part per 100,000).

The high-density (warm) and low-density (cool) spots on the CMB…

…are tiny quantum fluctuations that have been blown up in scale.

A dense region becomes denserdenser & more massivemore massive with time; its gravity

attracts surrounding matter.

Quantum fluctuations blown up by inflation are the “acorns”.

Great Oaks from Tiny Acorns Grow.

The Usefulness of Inflation

But is it correct?

Gravity Waves & Inflation

Inflation is predicted to have a strong influence on the spacetime of the early Universe

• Quantum overdensities – gravity pull• Expansion of spacetime itself

Disturbances in spacetime = gravity waves

These gravity waves will be very difficult to detect with the gravity waves detector such as LIGO.

BICEP2 results

Polarization of Light

Gravity Waves & the CMB

Gravity waves traveling at the time of recombination stretch & squeeze the space around electrons

Therefore, those electrons see more energetic photons come from one direction than another

Gravity Waves & the CMB

Remember: “hotter” photons have higher energy. Therefore, they “win” and their signal dominates

Polarization and Light

Polarization

Unfortunately, hotter photons also come from hotter regions in space!

So the signal is very difficult to detect and interpret.

However, the polarization patterns are different for these two cases

What Density does to CMB Polarization

The Signal!