Viewing the Big Bang Through Sunglasses

by Nicholas Mee on March 23, 2014

In 1929 Edwin Hubble revealed that distant galaxies are receding from us and that the more distant the galaxy the faster it is racing away.

Running the universe backwards, this means that the galaxies must have been much closer together in the past. It would appear that the universe and everything in it were compressed into a point in the distant past and that it has been expanding from this point ever since.

Part of the Hubble Extreme Deep Field image showing galaxies at distances up to 13.2 billion light years. The product of a 23-day exposure by the Hubble Space Telescope. (copyright NASA)

The beginning of the universe has a catchy name – the Big Bang – but it was christened by an ardent critic of the theory – the great astrophysicist Fred Hoyle. It is a great name, which is why it has stuck, but it is also rather misleading. The Big Bang is often represented as an explosion within the universe and this is definitely not correct. It suggests that the universe is a pre-existing container that held a cosmic egg from which the material that forms the stars and galaxies burst forth. This leads to the misconception that the Big Bang happened at a particular place. In fact, if the Big Bang happened anywhere, it happened everywhere at once. Every point in space is equally close to the Big Bang. We now know that the distance to the Big Bang is 13.8 billion light years. The idea is that the universe in its entirety – space, time and matter – began at the Big Bang.

It helps to consider the analogy of a balloon that is being blown up. The main difference is that the surface of a balloon is two-dimensional whereas space is three-dimensional. As the balloon expands, every point on its surface moves away from every other point and, the further apart two points are, the faster they recede from each other – just like the galaxies in the real universe. We can run the expansion backwards until every point on the balloon coalesces into a single point which represents the origin of this rubbery universe. From this perspective we can see that every point on the balloon universe is equally distant from its origin and, in fact, that the balloon Big Bang happened everywhere simultaneously.

Did the Big Bang Really Happen?

There is remarkably good evidence for the Big Bang. First, as Hubble discovered, the universe is expanding. It is also worth noting that the universe does not appear to contain any objects, such as stars, that are older than 13.8 billion years. This is a very important, and perhaps overlooked, consistency check on the Big Bang theory.

But there is also very good independent observational evidence. The universe would have been much hotter and denser in the past. For the first couple of minutes it would have been a nuclear furnace in which fusion reactions would have created deuterium (heavy hydrogen), helium and traces of other very light elements. (These conditions would not have persisted for long enough for any heavier atoms to be produced. All the heavier elements were generated much later in stars and supernova explosions.) It is possible to measure the amount of deuterium and helium and other elements in the universe and the observations closely match the amounts deduced from modelling the Big Bang.

Listening to the Noisy Soup!

Arno Penzias and Robert Wilson, two engineers working for Bell Labs, discovered the most compelling evidence for the Big Bang in 1964 when they were building a very sensitive new antenna in New Jersey. Their equipment was plagued with background noise which they initially assumed to be a fault in their equipment. They tried everything to eradicate the noise. Eventually, the explanation was provided by the Princeton astrophysicists Robert H. Dicke, Jim Peebles, and David Wilkinson who were preparing to search for microwaves from the early universe. Penzias and Wilson had discovered the cosmic microwave background. Its existence had been predicted as early as 1946 by the Russian physicist George Gamow.

So where do all these microwaves come from? After the era of nuclear synthesis, the expanding universe consisted of a plasma soup of charged particles composed largely of hydrogen and helium nuclei and free electrons. This soup would have been awash with photons – the fundamental particles from which light is formed. (Any other particles in the soup such as neutrinos would have gone on their merry way by this time and ceased to interact with the matter.) The photons would have bounced around continually scattering off the nuclei and electrons.

After around 380,000 years of expansion the matter would have cooled to around 3,000 degrees. It was now cool enough for hydrogen atoms to form. Prior to this any electron that combined with a proton to form an atom would immediately have been kicked out of the atom by a passing photon. Once the temperature had cooled below this temperature all the matter would have condensed into atoms and the photons would no longer have been able to interact with the matter. Just as hydrogen gas is transparent, so now the universe was transparent. It was still bathed in radiation and the photons forming this radiation had the energy and wavelengths corresponding to that emitted by matter at 3,000 degrees. This energy spectrum was frozen in as the photons could no longer interact with any matter.

These photons continued to race across the universe for billions of years and the universe continued to expand beneath their feet. These are the photons producing the noise that Penzias and Wilson could not escape. Each photon last interacted with an electron or other charged particle 13.8 billion years ago, just after the Big Bang and since then the universe has expanded in size by just over 1,000 times, so the wavelength of the photons has been stretched by a factor of over 1,000. What set out a vast distance away as a photon of visible light is now detected as a photon with a wavelength in the microwave range. This stretching means that the microwave background is now identical to the electromagnetic radiation that would be emitted by an object with a temperature of just 2.7 degrees above absolute zero – less than 1,000th of its original temperature.

Mapping the Early Universe

In 1989 NASA launched the probe COBE (Cosmic Background Explorer) in order to produce a detailed map of the cosmic microwave background. The result is shown below. The microwave background has an almost perfectly constant temperature of 2.726 K right across the whole sky. The map covers the entire sky showing regions that are very slightly cooler or very slightly warmer than average. Blue corresponds to 0.00002 degrees warmer and red corresponds to 0.00002 degrees cooler.

Map of the temperature variations in the cosmic microwave background as observed by COBE.

COBE was followed up by WMAP (Wilkinson Microwave Anisotropy Probe) which was launched in 2001. WMAP greatly increased the resolution in the measurements of the temperature variations in the microwave background and pinned down the age of the universe to 13.78 billion years. These very slight temperature variations correspond to very slight variations in the density of the universe just 380,000 years after the Big Bang. The denser regions are the seeds that would grow into clusters of galaxies as the universe evolved.

In 2008 the European Space Agency launched the Planck probe whose mission includes teasing out more information from the microwave background and refining the map produced by WMAP even further.

The Flat Universe Society

The evidence for the Big Bang all stacks up. However, the Big Bang model still throws up a few tricky posers.

If the universe is expanding like a balloon, then we would expect it to appear curved like the surface of our balloon, but the universe seems to be flat. Should we all join the Flat Universe Society? Perhaps not. We are familiar with the fact that when we survey our surroundings on Earth, the Earth looks flat in our vicinity, even though we know that the Earth is spherical. This suggests that the universe must be very much larger than the region that we can see.

When we look out into the heavens we are seeing light that set out on its way towards us long ago. But we can only peer out 13.8 billion light years in each direction. The light from any objects that are further away than this could not have reached us since the origin of the universe. This entire expanse of space looks flat, but it seems that when we gaze into the furthest depths of space we are seeing just a small portion of the cosmos. There is every reason to assume that space continues onwards well beyond the horizon.

Whose Been Heating My Porridge?

There is an even more perplexing issue. Imagine heating a bowl of porridge in a microwave oven. There may be lumps in the porridge and it may not be heated evenly, but if we leave it for a couple of minutes the heat will spread throughout the porridge until it is all at the same temperature.

Like our porridge, the universe is remarkably uniform from horizon to horizon. The cosmic microwave background is the same temperature throughout the sky. But the light that is arriving from one direction has travelled 13.8 billion light years to reach us and the light from the opposite direction has also travelled 13.8 billion light years. These two regions are almost 28 billion light years apart. Since the beginning of the universe insufficient time has elapsed for any radiation or other information to communicate between these two regions of the universe, yet they appear to have the same temperature. Unlike our porridge there should not have been enough time since the dawn of creation for the universe to achieve a uniform temperature.

This might sound like a minor philosophical conundrum. It is not the sort of problem that would keep most people awake at night. But cosmologists are light sleepers. Just like in the story of the Princess and the Pea, any lumpy piece of universe under the mattress will keep a cosmologist awake all night.


In 1981 an American cosmologist Alan Guth proposed a solution to these issues. He suggested that during the very very early epoch immediately after the Big Bang, for the tiniest fraction of a second the universe inflated exponentially. In a fraction of a second the universe was enlarged from being an infinitesimal speck to perhaps the size of a pea. It then continued to expand at a much more leisurely constant rate until reaching the size that we observe today. Guth suggested that his inflationary model would explain the flatness problem by enlarging the universe well beyond the horizon that we can see. It would also solve the uniform temperature issue because in his inflationary model the temperature could have equalized when the universe was just a mote in God’s eye before it underwent its exponential expansion. So is there any evidence for inflation?

Inflation is a theory of what happened in the first instant of creation. For several decades it appeared as though it might belong to the sort of cosmological speculation that could never receive any observational backing. But just last week there was some incredible news from a team of American scientists who are operating a telescope based at the South Pole. This experiment is known as BICEP2 (Background Imaging of Cosmic Extragalactic Polarization 2). It is a relatively small telescope (around 30 cm) that is observing the deep universe beyond our galaxy from Antarctica. The telescope is cooled to just 4 degrees above absolute zero, so that it can observe the cosmic microwave background .

The BICEP2 telescope in the Antarctic twilight. (copyright Steffen Richter, Harvard University)

Put Your Polaroid Glasses On Now!

The light that we receive from the Sun is randomly polarized, which means that the light waves are oscillating by the same amount in every direction. In other words, the light is unpolarized. Polaroid filters are formed of needle-like crystals that are aligned and embedded in a plastic film. The crystal needles will block light waves that are oscillating in the direction that is perpendicular to their alignment and will transmit the light waves that are oscillating parallel to their alignment.

A polarized electromagnetic wave. The blue arrows represent the direction of the electric field. The red arrows represent the perpendicular magnetic field. The electric field defines the plane of polarization of the electromagnetic wave.

Just as visible light can be decomposed into two polarizations, so can microwaves. BICEP has been specifically designed to examine the polarization of the cosmic microwave background. The results announced last week show that the microwaves are indeed polarized. The big question is: how did this polarization arise?

A map produced by BICEP2 of the cosmic microwave background polarizations in a region of sky in the southern hemisphere. The lines indicate the direction and degree of polarization.

Theorists predicted the observed polarization pattern. They believe that these polarized microwaves are the signature of inflation. More specifically, they believe that gravitational waves would have been generated during the inflationary epoch. The stretching and squeezing of space in this first instant produced by these gravitational waves would have distorted the material within the universe and left its imprint on the radiation that was emitted. In short, these distortions would lead to differences in the amount of light polarized in different directions.

The effect of a passing gravitational wave on the positions of a collection of test particles.

The results are consistent with an inflationary epoch that took place less than a trillionth of a trillionth of a trillionth of a second after the Big Bang.

Extraordinary claims require extraordinary evidence. Like all scientific discoveries this one will require corroboration from other experiments. Several other teams are currently exploring the microwave background. The BICEP telescope is looking at a relatively small region of sky. The Planck satellite is analysing the cosmic microwave background over the whole sky. The Planck team is due to report in October. Its results will be eagerly awaited. If everything pans out, then further analysis of the microwave background promises the possibility of extracting even more information about what was going on immediately after the Big Bang.


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