Feeds:
Posts

## Big Bang vs Steady State (part 1)

With the announcement earlier in the week of what appears to be direct evidence for cosmic inflation, I ended up getting involved in a discussion on one of John Gribbin‘s FaceBook posts with a gentleman who said “the Big Bang theory will be discredited in the next few years” (or words to that effect), and that the “Steady State theory” was the correct cosmological model.

I was a little surprised that there were even (presumably intelligent and informed) people who still felt that the steady state theory had any credence left. So, rather than answer this gentleman in private, I thought I would do a brief series of blogs on why we think that the big bang theory provides a more correct model of the Universe than the steady state theory.

I should remind readers (all two of you!), a theory is never complete. It is always a work in progress, and this is as true of the big bang theory as of any other theory. As Karl Popper said, it does not matter how many times a theory is confirmed, one robust refutation of that theory and it needs to be revised and/or abandoned. Cosmologists have been trying to test predictions of the big bang theory since Lemaître first proposed it in the 1920s, and they will continue to do so for the foreseeable future.

## The expanding Universe

The expansion of the Universe was observationally discovered by Edwin Hubble and his observing assistant Milton Humason in 1929. What Hubble and Humason found was that more distant galaxies appeared to be moving away faster than nearer galaxies. The recession velocity was determined by the Doppler shift in the spectral lines of the galaxies and was a pretty robust result. The distances were a little less robust, as there was no reliable way to determine the distances to the galaxies Hubble included in his study. However, since then we have been able to use the reliable method of Cepheid variables to determine the distances to a large number of galaxies. For example, the Hubble Space Telescope (named, of course, after Edwin Hubble) was able to observe Cepheid variable stars out to large distances in the 1990s. This was a Hubble Key Project. The relationship between the distance of a galaxy and how quickly it is moving away from us, the so-called Hubble law, is now well established.

Edwin Hubble (left) and Milton Humason, who discovered the expansion of the Universe.

In the 1910s Vesto Slipher, working at the Lowell Observatory in Flagstaff Arizona had found that from a sample of 25 “spiral nebulae” (as they were then known), 22 appeared to be moving away from us with 3 moving towards us, based on the Doppler shift in their spectral lines. Slipher noted that there was something strange about this, but never made the connection to an expanding Universe.

The diagram of distance (x-axis) versus recession velocity (y-axis) from Hubble’s original 1929 paper from the Proceedings of the National Academy of Sciences

Although Hubble himself never actually said it, the most natural interpretation of the Hubble law is that the Universe is expanding. It is not that the galaxies are rushing through space, but rather that space itself is expanding. A galaxy which is twice as far away as a given galaxy will move away twice as quickly if space is uniformly expanding. Naturally, if space is getting bigger then it would have been smaller in the past – so Hubble’s discovery lent natural support to the emerging idea of a Universe which started out small and is getting bigger.

In an expanding Universe, more distant galaxies move away quicker than nearer ones because of the expansion of space. The galaxies themselves are not moving through space, it is space itself which is expanding.

## Einstein’s biggest blunder

Einstein developed his General Theory of Relativity, a radically different approach to understanding gravity, in 1916. This theory describes gravity as a bending of space and time, rather than the classical idea of gravity that Newton had developed in 1666. In 1917, when Einstein applied his new equations to the Universe, he found that it predicted a dynamic (expanding or contracting) Universe. But, at the time the general consensus was that the Universe was static, so Einstein introduced a fudge-factor, the “cosmological constant”, to give his equations a static solution. When the expansion of the Universe was later discovered by Hubble and Humason, Einstein purportedly said that the cosmological constant was “the biggest blunder of my life”, as he could have predicted the expansion of the Universe some 12 years before hand.

## de Sitter, Friedmann and Lemaître

Two years after Einstein introduced his cosmological constant, in 1919, Dutch mathematician and physicist Willem de Sitter produced a solution to Einstein’s equations which had no matter but just the cosmological constant. This predicted an expanding Universe, but nobody took much notice as everyone knew the Universe contained matter.

In 1922, Russian cosmologist Alexander Friedmann produced the first solutions to Einstein’s equations which contained matter but which also predicted that the Universe might expand. Unfortunately for Friedmann, he died in 1925 and his work went largely unnoticed at the time, probably because he only published in Russian.

A few years later, in 1927, Belgian cosmologist and Catholic priest Georges Lemaître independently came up with the same idea as Friedmann. He was aware of Slipher’s work on the redshift of spiral nebulae, and conjectured that it might be a sign of the Universe expanding. He published his work in an obscure Belgian scientific journal, so it too was ignored. But then, renowned cosmologist Sir Arthur Eddington published a long commentary of Lemaître’s paper in the widely read Monthly Notices of the Royal Astronomical Society, propelling Lemaître’s work to prominence. Einstein became aware of Lemaître’s work, but was not convinced by it.

Then, in 1931, Lemaître published a letter in the most prestigious scientific journal, Nature, outlining his ideas on cosmic expansion in some detail. In this letter he suggested that the Universe had begun in what he called a primordial atom.

Newspapers around the World picked up on the story, and the New York Times ran a front page story with the headline

Lemaître suggests one, single, great atom, embracing all energy, started the Universe.

Einstein was won over, and in 1932 he and de Sitter published a paper of a model we now call the Einstein-de Sitter model, in which they stated that the correct cosmological model was one which would just about keep on expanding to infinity, but would take an infinite amount of time to do so and would never re-collapse.

In the 1940s a fierce opponent to Lemaître’s “primordial atom” theory would emerge, Sir Fred Hoyle. In part 2 of this blog next week I will talk about his competing theory, the “Steady State theory”, and Hoyle’s on-going battle in the 1940s and 1950s with George Gamow, who became the chief champion of the “primordial atom” theory.

## Studying the Universe using gravitational waves

The European Space Agency announced last week (28th of November) that a space-based gravitational wave observatory will form one of its next two large science missions. This is exciting news for Cardiff University, as it has a very active gravitational waves research group headed up by Professor B S Sathyaprakash (known to everyone as “Sathya”). The Cardiff group, along with others in the Disunited Kingdom, played an important role in persuading ESA to make this observatory one of its next large science missions. Just over a year ago I blogged about some theoretical modelling of gravitational waves the Cardiff group had done.

The ESA plan is to launch a space-based gravitational wave observatory in 2034, which will have much more sensitivity than any current or even future ground-based gravitational wave observatory. Although NASA also had plans to launch a space-based gravitational wave observatory, there is currently none in existence, so this is really ground-breaking technology that ESA is announcing. From what I can understand, the current announcement by ESA is their commitment to the proposed LISA (Laser Interferometer Space Antenna) gravitational wave observatory. It would seem NASA has withdrawn their commitment to what was originally going to be a joint NASA/ESA mission.

ESA has chosen a space-based gravitational wave detector to be funded as one of its two key future missions. It should go into operation in 2034.

## What are gravitational waves?

Gravitational waves are ripples in the fabric of space. They were predicted by Einstein as part of his theory of general relativity, the best theory we currently have to describe gravity. In his theory, events which involve extreme gravitational forces (such as two neutron stars orbiting each other (or merging), or the creation of a black hole) will lead to the emission of these gravitational waves. As they spread out from the source at the speed of light, they literally deform space as they pass by, just as ripples deform the surface of a pond as they spread out from a dropped stone.

An artist’s impression of gravitational waves being produced as two black holes orbit each other.

## Current gravitational wave observatories

There are several current gravitational wave observatories, all ground-based. These include VIRGO (in Italy) and LIGO (Laser Interferometer Gravitational Wave Observatory) which is in the United States. LIGO is the most sensitive of the current generation of gravitational wave detectors. LIGO actually comprises three separate detectors; one in Livingston, Louisiana and two in Hanford, Washington State. Each of the three separate detectors consists of two long arms at right angles to each other, forming a letter “L”. The idea behind these detectors is that, if a gravitational wave were to pass the detector, each of the two arms would have its length changed differently by the deformation of space as the gravitational wave passes through. Thus the detectors work on the principle of an interferometer, looking for tiny changes in the relative length of the two arms. And, when I say tiny, I mean tiny. In a 4km arm they are looking for changes of the order of $10^{-18} \text{ m}$, or about one thousandth the size of a proton!

The principle of a gravitational wave detector. They are essentially “interferometers”, with two arms at right angles to each other. As the gravitational waves pass the detector, space will be deformed and alter the relative lengths of the two arms.

## LIGO (Laser Interferometer Gravitational Wave Detector)

Currently the most sensitive gravitational wave detector is LIGO. LIGO consists of three separate detectors, one in Livingston, Louisiana and two in Hanford, Washington State. The detector in Louisiana is shown below.

The LIGO detector in Livingstone, Louisiana. Each arm is 4km in length, and can detect changes in the relative length of the two arms of less than the size of a proton.

The detector in Louisiana, and one of the two detectors in Washington State, consist of two 4km long arms at right angles to each other. An event like the collapse of a 10 solar-mass star into a black hole is expected to produce a change in length in a 4km arm of about $10^{-18} \text{ m}$, which is about one thousandth the size of a proton. This is just at the limit of the detection capabilities of LIGO, which is why astrophysicists are wanting more sensitive detectors to be placed into space. The other detector in Washington State has arms which are 2km in length, but just as sensitive as the detector with 4km arms at frequencies above 200 Hz, due to a different design.

## LISA – Laser Interferometer Space Antenna

The ESA plans just announced will be based on the NASA/ESA plans for LISA, which have been on the drawing board for most of the last 10 years. ESA’s plan is to build two space-based interferometers, which will be in the form of equilateral triangles as this artist’s description shows.

An artist’s impression of LISA, the “Laser Interferometer Space Antenna”. Each interferometer will consist of an equilateral triangle, with each side 5 million km in length.

The plan for LISA is to have arms which are 5 million km long! Compare this to the 4km long arms of LIGO. The changes in the length of a 5 million km long arm would be roughly one million times more than for LIGO, so rather than $10^{-18} \text{ m it would be } 10^{-12} \text{ m}$, which should be well within the capabilities of the detectors. This means that less energetic events than the collapse of a 10-solar mass star into a black hole would be detectable by LISA. All kinds of astrophysical events which involve large changes in gravitational fields should be detectable by LISA, including the afore-mentioned creation of black holes, but also the merging of neutron stars, and even the merging of less massive stars.

But, possibly most exciting is the opportunity that gravitational waves provide to probe the very earliest moments after the Big Bang. With normal electromagnetic radiation (light, x-rays, infrared light etc.), we can only see as far back as about 300,000 years after the Big Bang. This is when the Cosmic Microwave Background Radiation was produced. Prior to this time, the Universe was opaque to EM radiation of any wavelength, because it was full of unbound electrons, and the photons would just scatter off of them and not get anywhere (see my blog here about the CMB). But, it was not opaque to gravitational waves, so they provide a way for us to see back beyond the CMB, and a unique way to learn about the conditions of the Universe in its earliest moments.