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Redshift

As we look further and further out into space we see that the stars are more and more redshifted in color. The simplest explanation is that the universe is expanding in every direction. It is this expansion that produces a Doppler redshift in the wavelength of light.

Measurements are only approximate, but the rate of expansion, called the Hubble constant, or H, is normally taken to be about 50 to 100 kilometers/second per MegaParsec. A Parsec is 3.26 million light-years, so the constant may also be expressed as 2.4E^-18 meters/second per meter of distance. If we assume the rate of expansion has always been the same (that is, the same velocity at any point in time for any given current separation) then the inverse of the constant (with the dimension of time = seconds) will tell how far back in time everything was in the same place - the origin of the universe. The inverse of the constant gives about 13 billion years as its age, although a value of 15 billion years is also popular. Of course this assumes the rate of expansion is a constant, which would not be true for the Big Bang - it would be less now than earlier because of gravitational attraction, which would make the universe younger.

The Hubble constant also gives us the maximum possible size of the universe. The fastest two bodies can separate from us is light speed, so if we are in the middle of the universe the furthest away anyone can be from us is c/H, giving a radius of 13 billion light years. Spatial and temporal dilation means that if there are galaxies near this limit, and if the density of matter in any frame of reference in the universe is more or less constant, then the perceived galactic density will rise as we view near to this limit.

It is one thing to know that the universe is expanding, another to work out why. Here are two possible answers...

1. By inference with explosions on Earth, we can surmise that the universe started with a Big Bang and the expansion is the aftermath of that explosion. This is the most popular theory.
2. Einstein’s equations show as an obvious solution that the universe is expanding. He was unaware of the expansion of the universe as given by Hubble’s constant, so he added a cosmological constant to his equations to eliminate the expansion they predicted. There are other solutions to the equation, but the simplest and most probable one is the solution he found - the universe is expanding for no other reason than to satisfy its own equations for existence. It is not necessarily associated with an impelling force such as an explosion.

There is inferential proof of the Big Bang in the background microwave radiation that can be detected by suitable equipment as coming from every region of the universe. The explosion would have caused radiation that can be seen today as being incredibly redshifted and weakened in the microwave part of the spectrum. For this to be the case the universe must be closed so that no radiation leaks away - it does not work in a universe where the universe is bounded by an infinite void - and this is far from being proven and will remain unprovable for the foreseeable future. Even if it were bounded, more precise measurements of the spectrum are needed before it can be proved to be an aftermath of the Big Bang. So the jury is still out on the background radiation.

The big problem with the big bang theory is that there is no way it could have happened within the rules of our universe. So if you want the big bang theory to work you have to make up a special set of rules and assume they applied just for the duration of the big bang at the start of everything. Then after that the rules changed to what we know today. In some ways  it is another Creation Myth, just like the story of the spider Anansi that created the world - you take one solid piece of evidence (the perceived expansion of the universe), throw away what doesn’t fit your theory (the current rules of the universe, including the conservation of mass/energy), make up new and unprovable concepts (the “special” rules for the start of the universe), just to make the Big Bang work. Of course, this is the extreme opposing view.

More interesting is the work that shows that the rate of expansion of the universe may be increasing over time. This conflicts with the general Big Bang idea since the aftermath of such an explosion would result in a slowing down of expansion over time. It has to be said that the Big Bang itself violated all conservation laws, and in fact all theories of evolution of the universe must incorporate the violation of energy-conservation at some stage in the development of the evolving universe.

So there you have it. A Big Bang, or Einstein’s equation, or some combination, or something else altogether. The Big Bang wins hands-down by popular acclaim.

Quasars

Quasars are something of a puzzle. They are blue-looking objects with immense red shifts and luminous intensities 30,000 times brighter than the average galaxy, equivalent to the light from 10¹³ suns. The redshift can be as high as five. This is equivalent to saying (see above text on the Hubble constant) that the light we see was generated when the universe was 1/5^th of its current age, and the quasar is receding from us at 4/5^th of the speed of light. There were more quasars around in this early universe than there are today.

There may be quasars at even higher red shifts, but we cannot see them because of difficulties in viewing in the far infrared to which the wavelengths are shifted.

As the light from the quasar travels towards us it frequently encounters hydrogen clouds that absorb light according to the hydrogen absorption lines and to the redshift at which the hydrogen cloud lies from us. This confirms we are indeed looking back in time, rather than seeing some artifact we do not understand, and we can see from the final spectrum that hydrogen clouds were much more abundant in the early universe than they are now.

The nature of the light from quasars differs from that of the sun. It is continuous, with emission lines, particularly from Lyman alpha hydrogen transition. It is unstable on a time scale of days, implying an upper limit to the size of a few light-days across. By comparison the sun has a continuous blackbody radiation with gaps caused by absorption lines and is relatively stable.

No-one really knows what the answer is, but one picture that might fit is of a hydrogen cloud a few light-days across orbiting a massive central body which is exciting the cloud non-thermally. Some mechanism in the central body - intense rotating magnetic fields is a prime candidate - could be accelerating high-energy particles and smashing them into the cloud, creating an effect similar in style but not in scale to the aurora borealis. Black holes are often nominated as the central body, with an infall of matter equivalent to one solar mass per year. However, if the spectrum is one of excitation it may be a more efficient radiator than our sun; the sun puts out a lot of energy in the infrared, which may not happen here, so the infall needed may be a lot less.

If black holes do not exist the mechanism is still very similar. Hydrogen infall interacting with the rotating magnetic field of an ultra-massive neutron star might work. Modern neutron stars such as the one at the heart of the Crab Nebula have little hope of getting their hands on a hydrogen cloud, especially so soon after the supernova explosion - the debris is still blasting out from the core. But there was much more hydrogen around in the early days of the universe, and it concentrates in galactic centers.

Support for the concept of an ultra-massive neutron star remnant comes from the fact that most quasar emission lines come from elements such as carbon, oxygen, nitrogen and silicon, which fits in with the elemental debris left after a supernova explosion. Imagine an ultra-massive star formed with plenty of hydrogen near the core of a galaxy. If it is really large it will burn out rapidly and convection currents will be inadequate to mix the hydrogen in the outer layers with that of the core; the core burns out fast and it supernovas early, blowing away a lot of unfused hydrogen from the outer layers with some heavier elements, and shining intensely for many months. It cools down, but after many millennia friction in the galactic cloud combined with the gravity fields capture much of the debris and return it - together with plenty of fresh hydrogen - to the stellar remnant at the heart of the galaxy. The hydrogen infall starts and the quasar lights up the sky again.

If you are interested there is a lot of detail on the quasars on the internet.