Category Archives: Big Bang & Inflation

Inflation

History of the Universe - WMAP

Graphic for History of the Universe (Credit: NASA/WMAP Science Team)

The Big Bang theory found great success explaining the general features of the universe, including the approximate age, the expansion history after the first second, the relative atomic abundances from cosmic nucleosynthesis, and of course the cosmic microwave background radiation. And it required only general relativity, a smooth initial state, and some well-understood atomic and nuclear physics. It assumed matter, both seen and unseen, was dominating and slowing the expansion via gravity. In this model the universe could expand forever, or recollapse on itself, depending on whether the average density was less than or greater than a certain value determined only by the present value of the Hubble constant.

However, during the late 20th century there remained some limitations and concerns with the standard Big Bang. Why is today’s density so relatively close to this critical value for recollapse, since it would have had to be within 1 part in 1000 trillion of the critical density at the time of the microwave background to yield that state? How did galaxies form given only the tiny density fluctuations observed in the microwave background emitted at the age of 380,000 years for the universe? And why was the microwave background so uniform anyway? In the standard Big Bang model, regions only a few degrees away from each other would not be casually connected (no communication even with light between the regions would be possible).

There are four known fundamental forces of nature. These are electromagnetism and gravity and two types of nuclear forces, known as the strong force and the weak force. Physicists believe all the forces but gravity unify at energies around  10,000 trillion times the rest mass-energy (using E = mc^2) of the proton (1 Giga-electron-Volt). At some point very early in the life of the universe, at even higher energies equal to the Planck energy of 10 million trillion times the proton mass, all of the four forces would have been unified as a single force or interaction. Gravity would separate from the others first as the universe’s expansion began and the effective temperature dropped, and next the strong force would decouple.

We also must consider the vacuum field, that represents the non-zero energy of empty space. Even empty space is filled with virtual particles, and thus energy. At very early times the energy density of the vacuum would be expected to be very high. During the very earliest period of the development of the universe, it could have decayed to a lower energy state in conjunction with the decoupling of the strong force from the unified single force, and this would also have driven an enormous expansion of space and deposited a large amount of energy into the creation of matter.

In the inflationary Big Bang model postulated by Alan Guth and others, the decay of the vacuum field would release massive amounts of energy and drive an enormous inflation (hyperinflation really) during a very short period of time. The inflation might have started one trillionth of one trillionth of one trillionth of a second after the beginning. And it might have lasted until only the time of one billionth of one trillionth of one trillionth of a second. But it would have driven the size of the entire universe to grow from an extremely microscopic scale up to the macroscopic scale. At the end of the inflation, what was originally a tiny bubble of space-time would have grown to perhaps one meter in size. And at the end of the inflationary period, the universe would have been filled with radiation and matter in the form of a quark-gluon plasma. Quarks are the constituent particles of ordinary matter such as protons and neutrons and gluons carry the strong force.

The doubling time was extremely short, so during this one billionth of one trillionth of one trillionth of a second the universe doubled around 100 times. In each of the 3 spatial dimensions it grew by roughly one million times one trillion times one trillion in size! This is much greater than even Zimbabwe’s inflation and happens in a nearly infinitesimal time! The inflationary period drove the universe to be very flat topologically, which is observed. And it implies that the little corner of the universe we can observe, and think of as our own, is only one trillionth of one trillionth of the entire universe, or less. There is good observational support for the inflationary Big Bang model from the latest observations concerning the flatness of the universe, given that the mass-energy density is so close to the critical value, and also from the weight of the evidence concerning the growth of original density fluctuations to form stars and galaxies.


The Big Bang model

CMB spectrum (COBE)

Cosmic Microwave Background spectrum (credit: NASA)

The Big Bang theory describing the origin and expansion of the universe from a very tiny and energetic initial state was developed initially in the 1920s as a solution for Einstein’s equations of general relativity. It assumed, correctly, a uniform (homogeneous) density of matter and energy. While the universe around us today appears highly non-uniform, with visible matter apparently concentrated in groups of galaxies, and in individual galaxies, gaseous nebulae, and star clusters, stars, and planets, all the evidence indicates that matter was very uniformly distributed throughout the first one million years of existence. At that time there were no stars or galaxies, rather the universe consisted of hot dense, but expanding, gas and photons (light). Even today, on the largest scales of 500 million light years and beyond, the universe appears to be quite uniform on average.

The first great support for the Big Bang came from the detection of what we call the Hubble expansion, named for Edwin Hubble, who in 1929 first demonstrated that galaxy recession predominates and depends on distance from us. Galaxies on average are all moving away from each other, unless they are gravitationally bound to their neighbors. The rate of expansion is simply proportional to the distance to the galaxy; this is known as Hubble’s law. Every galaxy moves away from every other galaxy regardless of its position in the universe; this implies a global and uniform expansion.

How do we determine this relationship? The light from these distant galaxies is shifted to be redder than normal in proportion to the velocity away from our galaxy. The redshift is a measure of the velocity of recession and the velocity is found to be proportional to the distance from our Milky Way to the galaxy in question. To be clear, the galaxy velocity and distance follow a linear relation. If we were located in another galaxy, we would observe the same effect. Most of the galaxies would be receding from us as well, at rates proportional to their distance. This is just what one expects for a universe which is isotropic – the same in each direction – and which is expanding uniformly. Each dimension of three-dimensional space is getting larger with time. The gravitationally bound objects, such as the galaxies themselves, are not expanding, but the space between the galaxies is stretching and has been since the Big Bang initial event.

Since the rate of the expansion is proportional to distance, one can take the proportionality constant, known as Hubble’s constant, and by inverting that determine an approximate age of the universe. It amounts to ‘running the movie backward.’ The age works out to 14 billion years, which is very close to the current best estimate of the age of 13.8 billion years, about 3 times the age of the Sun and the Earth.

Another great success of the Big Bang model was in its prediction of the helium abundance. The same hydrogen fusion process that powers the Sun took place in the early universe during the first 20 minutes, when the temperature was millions of degrees. In the Sun hydrogen is fused to created helium. For the early universe, this is known as primordial or Big Bang nucleosynthesis. There was only time enough and the right conditions to create helium, the second lightest element in the periodic table, and also the heavy form of hydrogen known as deuterium, plus just a bit of the third element lithium. None of the heavier elements such as carbon, nitrogen, oxygen, silicon or iron were created – this would happen later inside stellar furnaces. The final result of this cosmological nucleosynthesis turned 25% of the initial available mass of hydrogen into helium, and into trace amounts of deuterium, lithium and beryllium. The primordial abundance observed in the oldest stars for helium and deuterium matches the predictions of the Big Bang nucleosynthesis model.

The Big Bang moved from being possible theory to well-established factual model describing the universe when the first detection of the cosmic microwave background was published in 1965 by Arno Penzias and Robert Wilson, who received the Nobel Physics prize for their discovery. The cosmic microwave background is blackbody thermal radiation at millimeter wavelengths in the radio portion of the electromagnetic spectrum., and as we observe it at present, it has a temperature of a little under 3 degrees above absolute zero (see image above which has the characteristic thermal blackbody shape). It fills space in every direction in which one observes, and is remarkably uniform in intensity. The cosmic microwave background dates from a time when the universe was about 380,000 years old, and the radiation was originally emitted at a temperature of around 3000 degrees on the Kelvin scale. It also has redshifted, by over 1000 times. Thus we detect today as radio waves photons that were originally emitted in the optical and infrared portions of the electromagnetic spectrum when the universe was only 380,000 years old. Unlike the hydrogen and helium atoms which are found in stars and on planets, these photons have stretched out in proportion to the expansion of the universe.


Scale of the universe

Hubble Ultradeep Field

Hubble Ultra Deep Field

Until 500 years ago the premise of an Earth-centric solar system and universe prevailed. And until 100 years ago it was thought that we lived within the confines of a single galaxy, our Milky Way. But in 1915 Albert Einstein introduced general relativity, the highly successful theory of gravity which couples mass, energy and the geometry of space-time. In the 1920s Alexander Friedmann and Georges Lemaitre introduced solutions to the equations of general relativity for an expanding universe. Lemaitre’s work indicated distant galaxies would have their light shifted to be redder than that of nearby galaxies. And by 1929 this was observed by Edwin Hubble. Now with the Hubble Space Telescope we can observe galaxies at much greater distances than Hubble could over 80 years ago. The image above is a very long exposure from the Hubble Space Telescope revealing close to 10,000 galaxies; many of these are billions of light-years away.

Hubble essentially measured the rate of expansion of the universe at the present epoch. The universe is expanding and galaxies are generally receding from one another except when they are gravitationally bound to their near neighbors. The value for the rate of expansion has been refined over the intervening years but is now accurately measured and indicates an age of just under 14 billion years for our universe.

The size of the universe as a whole we are unable to measure! We are limited by our own horizon, due to the finite speed of light. Only galaxies apparently moving away from us at less than the speed of light are within our horizon (also known as light cone). General relativity allows for space itself to stretch at faster than the speed of light if the separations between two galaxies are large enough; objects do not travel faster than light speed within their own local frame.

Our own observable portion of the universe has a lookback time distance of 14 billion light-years and what is known as the comoving distance of nearly 50 billion light-years. The comoving distance takes into account the expansion of the universe as the light moves through it from the Big Bang until now.

Note from the table below how much larger the universe is than the distance to the center of our galaxy or to the nearest star.

Object                    Distance (light travel time)

Nearest Star                        4.2 years
Center of Milky Way         25,000 years
Andromeda Galaxy           2.5 million years
Oldest Galaxies                  13 billion years
Big Bang                              13.8 billion years