Monthly Archives: March 2011


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.


Dark Energy Survey

DES logo

Dark Energy Survey logo

The Dark Energy Survey (DES) is a ground-based cosmology experiment led by astronomers from the US, Brazil and Europe. It has begun its trip to Chile where it is scheduled to begin observations in November, 2011 using the 4 meter Victor M. Blanco telescope in the Atacama desert. It uses a new highly sensitive camera design called DECam, with resolution totaling 570 Megapixels and employing very large pixels, and it emphasizes sensitivity in the red and infrared portions of the spectrum, in order to measure galaxies out to redshifts of 1 and beyond. Galaxies in the early universe are far away from us and have high redshift values. Light which they would have originally emitted in the blue or yellow portions of the optical spectrum has shifted toward the red or infrared, thus the emphasis on detection of infrared photons for this work.

The DES uses a 4-pronged attack to improve the measurement of the dark energy and other cosmological parameters. These 4 tests are:

  1. Supernovae – Type 1a supernovae are thought to occur when a white dwarf in a binary stellar system accretes mass from its companion. Once enough mass is accreted, the white dwarf is pushed over the Chandrasekhar limit of 1.4 solar masses, the gravity of the star’s mass overwhelms the pressure support from its ‘degenerate electron’ matter, and the white dwarf undergoes core collapse and becomes a supernova. It is temporarily as bright as an entire galaxy. Such a supernova can be detected at large distances (high redshifts) and very importantly, since the mass of the supernova is always the same, the absolute brightness of this type of supernova is essentially expected to be the same as well. This allows us to use them as standard candles for distance measurement and thus for cosmological tests.
  2. Baryon acoustic oscillations – This test looks at the statistics of galaxy separations at very large scales. In the early universe, sound waves were established in the hot dense plasma, reflecting pressure generated by the interaction of photons and ordinary matter. Dark matter does not participate except gravitationally. A “sound horizon” is expected with a present size of about 500 million light years, and this acts as a standard ruler as the universe expands. A bump in the correlation function, which measures the probability of one galaxy being near another, is expected at this characteristic distance.
  3. Galaxy cluster counts – This test of how many galaxy clusters are detectable versus redshift was apparently first proposed by myself in 1980, in the context of X-ray emission from the very hot diffuse gas found between galaxies in galaxy clusters. This approach offers certain advantages in comparison to simple galaxy counts versus redshift. In this case it will be performed in the infrared and red, observing the galaxies themselves. Galaxy clusters contain up to 1000 or more galaxies within a single cluster. The number of clusters that can be seen at a given redshift is dependent on the cosmological model and the mass of the cluster, since dark matter promotes cluster formation through gravitational attraction. Dark energy inhibits cluster formation, so this helps to measure the relative strength of dark energy at earlier times. The team expects to detect over 100,000 galaxy clusters, out to redshifts of 1.5.
  4. Weak lensing – This refers to gravitational lensing. This occurs when a source galaxy is behind an intervening galaxy cluster and the gravity of the cluster bends the light from the source galaxy in accordance with general relativity. By surveying a very large number of galaxies, a strong statistical measure of this bending, also known as cosmic shear, can be taken. The amount of shear will be measured as a function of redshift (distance). This shear is sensitive to both the shape of the universe and the way in which structure develops over time.

More info:

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.