The Quest for Cosmic Inflation

Big Bang Needs Tweaking

The Big Bang theory offered three important experimentally verifiable predictions:

  • The abundance of light elements during the first few minutes after the Big Bang (heavier nuclei were produced much later in the interior of stars).
  • CMB as a leftover of the intense heat from the Big Bang and the shape of its spectrum.
  • Hubble’s observation that as the universe expands, galaxies recede from one another with a velocity proportional to the distance between them.

The central ideas of the Big Bang (a) the expansion (b) the early hot state (c) the formation of light elements, and (d) the formation of galaxies—are derived from these and other observations. It is also safe to conclude the galaxies were closer together in the past since the distance between galaxies keep increasing today. The earliest instant of the Big Bang expansion is still an area of intense research.

But in spite of its striking success in explaining a host of observations, there were some puzzling aspects. The Big Bang theory provided no explanation for the universe’s surprising uniform temperature distribution and flatness. The following are generally considered to be the main outstanding problems with the Big Bang theory.

Horizon problem: Different regions of the universe have the same temperature and other physical properties although they have not “contacted” each other because of the great distances between them. This is surprising given that the transfer of information (or energy, heat, etc.) can occur, at most, at the speed of light. Light has a finite velocity and can only have traveled a finite distance since the Big Bang. As a result, at every point in the universe there is a sphere with a radius of 14 billion light years, called the visible universe.

The horizon problem can best be explained by taking a closer look at the CMB spectrum—the CMB spectrum has been measured with progressively greater precision in COBE, Wilkinson Microwave Anisotropy Probe (WMAP), and Planck satellite missions. As we have seen before, the CMB is very nearly isotropic. This can happen if the different regions of the sky could have interacted in the past and been driven towards thermal equilibrium. However, consider two points in the CMB on opposite sides of the sky; they are separated by almost 28 billion light years, far longer than light has been allowed to travel since decoupling 380,000 years after the Big Bang. If no signal can go faster than the speed of light, it is impossible they could have communicated with each other during the lifetime of the universe. The Big Bang model had no answer for the fact that the universe has the same temperature throughout.

The horizon problem has been explained beautifully using an analogy by Janna Levin in her book How the Universe Got its Spots: “If an anthropologist were to discover that two ancient civilizations had identical languages with only one word in a hundred thousand distinguished, they’d surely argue for a causal explanation. The two civilizations must have been in contact and the scientific task would be to explain how they had communicated”. We will see in the next Section how Alan Guth’s Inflation theory did this for cosmology and managed to get rid of the horizon problem that was plaguing the Big Bang model of cosmology.

Flatness problem: The universe appears to have a flat geometry. In Section 3 we discuss the geometry and fate of the universe using the cosmological parameter \sigma_0, a dimensionless parameter that describes the ratio of the actual mass density of the universe to the critical mass density. A universe with \sigma_0 = 1 is globally flat, at the borderline between a closed, finite universe, and an open, infinite universe. In other words, the actual mass density of the universe is very close to its critical density. The problem is that for the universe to be so close to critical density after about 14 billion years of expansion and evolution, it must have been even closer at earlier times because if the universe had begun with a slight positive or negative curvature, then this curvature should increase over time, not go away. The Big Bang model provided no explanation for the density of the universe to be so close to its critical density—a strange coincidence indeed.

Matter-Antimatter asymmetry: Another profoundly puzzling question is why the universe is made of matter rather than antimatter? Physicists currently believe just after the Big Bang copious amounts of the matter and antimatter were created in equal amounts. They destroyed each other promptly but a small amount of matter surplus formed after all these annihilations. This surplus is what makes up all the galaxies, stars and planets we have today.

Scientists are searching for asymmetries in the ways matter and antimatter behave although the possibility of such asymmetries for most types of particles has been ruled out. The only exception are neutrinos and there is mounting evidence pointing to neutrinos as the origin of this asymmetry. If neutrinos are not their own antiparticles (or not Majorana particles 9), then perhaps neutrinos could oscillate to other flavors 10 at different rates than antineutrinos do. These different rates, then, could have caused more neutrinos to survive the era of matter-antimatter annihilation in the early universe.

Neutrinos are fascinating particles that are perhaps the least understood. They are everywhere in the universe, but we cannot see them or feel them nor can we stop them most of the time. They were created in abundance during the Big Bang, and stars like our sun produce them copiously all the time. Neutrinos and the question of matter-antimatter asymmetry are topics too big to cover in one article. They are the subject of the essay Neutrinos.

Also surprising was the fact that the universe, having started in a very chaotic fashion, quickly morphed into a state where it became homogeneous and isotropic in every direction. Today, clusters of galaxies are pretty evenly distributed at the largest scale. The temperature of CMB differs by only 0.00001 across great swaths of space separated by about 28 billion of light years. It was quickly realized the Big Bang theory needed some refinements to account for the puzzling observations described above. These improvements came in the form of a new theory henceforth known as Inflation Cosmology which the subject of the next Section.


9 Particles and anti-particles are identical except for their helicities or handedness On the other hand, Majorana particles are distinct from their anti-particles.

10 Neutrinos are available in three different types, or flavors (electron neutrino, muon neutrino, and tau neutrino), and while in transit they can oscillate, or change their identities, between these flavors.

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