Inflation

Problems of the Universe

1. Why is the Universe expanding?
2. Why is the Universe at large so smooth (as evidenced by tiny fluctuations in the Cosmic Microwave Background)?
3. Why is the Universe so flat (as measured from WMAP)?
4. The horizon problem: How can regions of the Cosmic Microwave Background more than 1 degree apart, which were causally disconnected at the time of Recombination, know to have the same temperature today?
5. What caused ripples in the smoothness?
6. Where did matter in the Universe come from?

   Remarkably, the theory of Inflation offers a solution to all of these problems.

WMAP Wilkinson Microwave Anisotropy Probe (2008)

WMAP total

WMAP including dipole

WMAP fluctuations

Inflation is a hypothesized epoch in the very early Universe when the mass-energy of the Universe was dominated by vacuum energy.

• Vacuum energy is the quantum zero-point energy of the vacuum.
• Vacuum energy is gravitationally repulsive, according to General relativity.
• Consequence: the Universe inflates, doubling in size each "tick of the clock".
  If the vacuum energy was that of GUT (Grand Unified Theories; see below), then
  • each tick was about 10^-35 seconds long;
  • inflation took place over perhaps 10^-33 seconds altogether;
  • the Universe grew from perhaps a Planck size, 10^-33 meters, to perhaps a meter in size;
  • the temperature was about 10²⁴ Kelvin;
  • the vacuum density was about 10⁸⁰ kg/m³.

Solution to Problem 1: Why is the Universe expanding?

• The repulsion of the vacuum accelerated the Universe into tremendous exponential expansion – inflation.

Solution to Problem 2: Why is the Universe so smooth?

• Any initial fluctuations in density were stretched (redshifted) into smoothness.

Solution to Problem 3: Why is the Universe so flat?

• Inflation made the Universe so huge that today we see only a small patch of it, which looks flat. On a much huger scale, well beyond our horizon, the Universe might not be flat at all.

Solution to Problem 4: The horizon problem.

• Before inflation, the different parts of the Universe were causally connected.
• Inflation caused different parts of space to accelerate away from each other, eventually faster than the speed of light, so that they became causally separated.
• So the solution is: Causally disconnected regions of the Cosmic Microwave Background were once causally connected, before Inflation.
• The observed peaks in the power spectrum of fluctuations in the Cosmic Microwave Background require that the fluctuations were initially synchronized (like the initial pluck of a guitar string), as predicted by inflation.
• Probably the most compelling observational reason in favor of inflation.

WMAP Power Spectrum (2008)

Solution to Problem 5: What caused ripples in the smoothness?

• Tiny quantum fluctuations in the inflationary vacuum energy produced ripples,
• which later grew by gravity into galaxies, stars, and you.
• Inflation predicts a "scale-invariant" power spectrum at the largest scales, in agreement with the observed WMAP power spectrum.

Solution to Problem 6: Where did matter in the Universe come from?

• The observed vacuum energy today is tiny, about 10^-26 kg/m³, far less than the 10⁸⁰ kg/m³ GUT density.
• Therefore, inflation must have ended at some point.
• Theory:

  Vacuum energy  ®  Matter + radiation energy

What is this vacuum energy that drove inflation?

• It's energy associated with the Unification of forces – specifically, with the unification of the electroweak and strong forces into a hypothesized GUT (Grand Unified Theory) force.
• The interactions of all particles appear to be governed by just four forces:
  • Gravity
  • Electromagnetism
  • The strong, or nuclear, or color force
  • The weak force
• Weinberg and Salaam won the 1974 Nobel prize for their theory of the unification of the electromagnetic and weak forces into a combined electroweak force.
  • Electroweak unification appears only at high energy, above about 10¹⁵ Kelvin, which is about the highest energy accessible in particle accelerators today.
  • Electroweak unification correctly predicts various experimentally observed phenomena – such as that, at high energy, neutrinos (which interact only by gravity and the weak force) interact similarly to electrons (which interact by gravity, the weak, and electromagnetic forces).
• Extrapolating, physicists predicted that at even higher energies the electroweak force may unite with the strong force into a single GUT force.
  • But the GUT energy is too high, about 10²⁸ Kelvin, to be reached in particle accelerators, so the theory is not well understood and is not experimentally tested.
• The (de)unification of forces is like a phase transition (like steam « water). Vacuum energy is the "latent heat" of the transition.

  Schematic diagram

  Genuine physics diagram The running coupling constants as function of energy in the Standard Model and the Minimal Supersymmetry Extension of the Standard Model.

In summary, inflation offers an enticing and observationally consistent picture that suggests how the Universe today could have come from almost "nothing".

• Inflation does not explain how the laws of physics that govern our Universe were laid down, nor why those laws are so beautifully adapted to life.

What happened before inflation?

• Unanswered mystery.
• Were the GUT and gravititational forces unified into a "Superforce"?
• Since gravity is the force associated with the nature of spacetime itself, could time itself have started when gravity split off from the other forces?
• Did M-theory (supersting theory), a currently favored mathematical TOE (Theory Of Everything), which predicts that spacetime should be 11-dimensional, prevail?
• Are baby universes made inside black holes?

Updated 2009 Apr 22