Big Bang

The big bang is the widely scientifically accepted theory of how the universe came into being (the actual cause of the big bang itself is still unknown and is possibly unknowable). According to one version of the theory, all of space and time (spacetime) "began" about 13.7 billion (± 200 million) years ago, and has been expanding ever since. Another version of the theory is that a hypothetical "multiverse" existed before our universe began. While many details of the theory remain to be worked out, especially involving the first few instants of time, the big bang model is supported by many converging lines of evidence. The pillars of the big bang theory:

• Predictions of Einstein's General Relativity
• Observed expansion of the universe
• Observed cosmic microwave background radiation
• Observed ratios of elements left over from the early universe

It is important to note that the big bang theory does not attempt to describe the initial conditions or first cause of the universe. It is intended to describe the development of the universe from its extremely dense and hot early stages into its present form. It is instructive not to think of it as a localized explosion from which all matter moves away, but rather as a uniform expansion of space itself. An observer at any point in the universe sees the same thing: a homogenous distribution of matter everywhere, with the more and more distant parts receding faster and faster.

Starting assumptions

There are exactly two assumptions required to construct the Big Bang--and neither requires God (yeah, imagine that--although it should be noted that God isn't included because the evidence doesn't require one, not because the evil scientist conspiracy needs to destroy supernatural powerz). In fact, there is empirical evidence for each of these assumptions, so they should be considered reasonable, defensible statements rather than postulates.

1. The laws of physics are the same everywhere in the universe.
2. On a sufficiently large scale, the universe is homogeneous and isotropic.

That's it. Really. The first assumption is pretty straightforward, because a.) we haven't seen evidence to the contrary despite looking into space for a pretty long time, and b.) without it you might as well give up on doing any astronomy, astrophysics, and cosmology at all, since if physical laws in the Andromeda galaxy are somehow different from where we live, but the differences are so subtle that we can't detect any from where we are...well, it's pretty hard to go there and measure them. This assumption is necessary because when talking about how things interact on galactic, much less universal, scales, you need to use General Relativity. It's much better if General Relativity applies to other galaxies in the same way that it applies to ours.

The second assumption is known as the cosmological principle and has strong empirical support. It's essentially a stronger version of the Copernican principle, which says that the Earth has no special place in the cosmos.

Evidence for the big bang

There are four primary pieces of evidence for the big bang that are so well-established that they are referred to as the "four pillars" of the big bang. While other pieces of evidence exist, these four are the most compelling.

Pillar 1: The universe is expanding

See the main article on this topic: expanding universe

Edwin Hubble's observations and analysis in the late 1920s showed that the farther away a galaxy is from our own, the faster it is receding from us. You are probably not surprised to learn that the relationship between a galaxy's distance and recessional velocity is known as Hubble's Law. There are two possible explanations for these observations.

1. The Earth is at the center of a massive explosion of galaxies.
2. The universe is uniformly expanding.

Explanation 1 is untenable because it is in conflict with the cosmological principle (see above starting assumptions). That leaves explanation 2.

If we extrapolate the consequences of Explanation 2 into the past, we discover that all the matter in the observable universe would have been very close together about 14 billion years ago.

Pillar 2: Cosmic microwave background radiation

If the matter in the early universe was highly compressed, it would have been extremely hot and dense--so much so that atoms couldn't form, and there was simply a sea of electrons, protons, and photons. The photons would constantly interact with the electron-proton plasma, constantly forming and annihilating without going very far. Once the universe cooled enough to allow electrons and protons to combine into hydrogen atoms, the remaining photons were "released," meaning they could travel large distances as radiation without interacting with a charged particle. Thus, if the Big Bang occurred, we should see vestiges of this radiation permeating all space, and it should look the same in all directions. The radiation should also follow a black body spectral pattern.

Furthermore, the radiation would have been very highly energetic, with a very short wavelength, at the time of recombination. However, the universe's expansion since that time would have lengthened the wavelength of that radiation and cooled it considerably. Over time, the radiation would transition from X-ray to ultraviolet to visible (yikes, good thing our eyes didn't exist then) to infrared to microwave.

Hey, guess what we observe today? An isotropic, black body spectrum of radiation peaked in the microwave region of the spectrum, with a temperature corresponding to 2.726 Kelvin.

Pillar 3: Abundance of light chemical elements

Remember that stuff above about how the universe cooled to a point where hydrogen atoms could form? There's a reason why hydrogen would form first--it's just a proton bound to an electron, the simplest way to make an atom. Slightly more complex atoms could form after that, but if you're starting with a soup of electrons and protons it's not very energetically favorable to make a huge atom such as, say, lead instead of hydrogen and helium. As a result, if the Big Bang occurred, we would predict the universe to consist of tons of hydrogen, some helium, a little bit of heavier things like lithium and deuterium and other isotopes, and not much else (heavier atoms would have to wait for stars and supernovae to be fused).

Hey, guess what the composition of the observable matter in the universe looks like? Observationally, it's 74% hydrogen and 24% helium. Not bad. Even more cool, you can predict relative abundances of this matter using a single parameter, the photon to baryon ratio. How do you know what the photon to baryon ratio is? If you like, you can figure it out by measuring tiny fluctuations in the cosmic microwave background radiation. If you do, you'll find that the elemental ratios you arrive at are extremely close to those observed spectroscopically. Not bad at all.

Pillar 4: Galactic morphology and distribution

The distant galaxies from us are many light years away, so when we observe them, we are seeing them as they were long ago due to the light travel time. Consequently, we can get pretty good ideas about star formation, galaxy formation, galaxy cluster formation, and supercluster formation because we can see snapshots of these things happening at different eras. It turns out that galaxies that formed long ago are quite different from the nearby ones that we see today, as measured by star and quasar formation.

These observations suggest that the universe was different in the past than it is now, which is evidence against the "steady state model" of the universe that was an alternative to the Big Bang before the cosmic microwave background radiation was discovered. These days pretty much all real scientists acknowledge that the Big Bang is the way to think about the universe.

Inflation

One of the biggest problems with the original big bang theory is the extreme homogeneity of matter in the universe; the density of galaxies and gas clouds is the same no matter which direction we look. Scientists like Professor Alan Guth solved this problem, known as the horizon problem, by introducing the concept of inflation - shortly after the initial moment, the universe underwent a period of rapid expansion which smoothed out the density fluctuations. While it explains a few things, inflation leaves even more questions - most prominently, "what force of nature could cause that?".

Chaotic inflation theory, or bubble universe theory, is an alternative model of inflation. Developed by physicist Andrei Linde and others in 1986, it solves a problem of the inflation theory, namely how to end the inflationary period.

Creationists

Of course, any time two scientists disagree about a minor aspect of the theory, the conversation is mined for any quotes which could be misrepresented to support creationism. However, any discussion about the development of the universe is bound to be severely limited - the whole of our observations are made from one tiny corner of space, in the blink of an eye. We are not done learning yet.

How to create a universe

Dr. Guth and others hope to figure out how to create a universe in the laboratory. Guth once stated in an interview:

I in fact have worked with several other people for some period of time on the question of whether or not it's in principle possible to create a new universe in the laboratory. Whether or not it really works we don't know for sure. It looks like it probably would work. It's actually safe to create a universe in your basement. It would not displace the universe around it even though it would grow tremendously. It would actually create its own space as it grows and in fact in a very short fraction of a second it would splice itself off completely from our Universe and evolve as an isolated closed universe growing to cosmic proportions without displacing any of the territory that we currently lay claim to.

Question

"What was there before the big bang?"

One possible answer is:

"The question is nonsensical, because there was no time (or space) for anything to exist in, so the word before is meaningless."

Another possible answer is:

"The multiverse existed before the Big bang.”

As both the above are non-intuitive and go counter to all everyday human experience, it is the major point upon which popular understanding fails.

Salvation, for one editor at least, came with the Planck Time. The Planck Time is the shortest meaningful length of time. It is somewhere around 10⁻⁴³ seconds, which is extremely short, but not zero. It is not possible to know what happened less than one Planck Time after the big bang. Indeed, it is not just not possible to know what happened, it is actually meaningless to even ask the question. That being the case the question of what happened before the big bang is also meaningless. We just have to lump it and get on with asking questions which are meaningful.

Hawking's book A Brief History of Time gives a reasoned explanation of the big bang and subsequent events, but is popularly reckoned to be intensely dense to the point of unreadability. Another book A Briefer History of Time has since been published.

Julian Barbour suggests that reality simply terminates on nothing at the alpha point, as a brute fact, in the same way that England abuts the sea at Land's End without requiring an explanation.

Theory of everything?

It is notable that the disciplines of quantum physics, relativity and astrophysics all converge in the big bang theory - a first for science.

Addendum: God?

Theists of all stripes have attempted to use the theory as a "proof" of the existence of God. Well Goddidit:

"How was the big bang initiated, if not by a supernatural being?" they ask.

To which there is only one reply necessary:

"How was the supernatural being created if not by a supernatural being?"

And so on ad infinitum.

This paradox is unsolvable, so in the end, it comes down to a question of faith, or lack thereof.

In A Brief History of Time, Stephen Hawking outlines the mathematical use of imaginary time which results in the description of the universe as being of a hyperspherical nature without start or end - these being merely points on a "surface" undistinguished from others. The upshot is that the requirement for "start" and "cause" are removed, as is the need for faith (a concept which has no place in science).

Interestingly enough, the big bang theory was first proposed by Catholic priest and professor of physics Georges Lemaître. He first brought the theory to public attention after the discovery of redshift of nearby nebulae, although it was Fred Hoyle who coined the actual name as a derisory term. Compared with current big bang theory, which incorporates aspects such as inflation, LeMaitre hypothesized that all matter for the universe came forth from a "primeval atom", today more commonly described as a singularity.

So what happens next?

The universe, from this point, could:

1. Keep expanding, until it expands so far it cannot collapse back upon itself. Eventually matter and energy would be so spread out that no particles would interact again. This is called "the heat death of the universe".
2. Keep expanding, but reduce the rate of expansion by a fraction, eventually reaching nearly 0.
3. Keep expanding, but at slower and slower rates, until gravity takes over, compressing all the mass back into a singularity, perhaps kicking off another big bang.

A fourth option, which was only recently discovered, is that the expansion will keep accelerating until the universe is torn apart at the atomic level. Recent studies into the cosmic microwave background radiation, gravitational lensing, and, most importantly, improved measurements of supernovas have led to the discovery that expansion really is accelerating. A possible explanation for this acceleration is the fact that, as the universe expands, the density of dark matter decreases while the density of dark energy remains constant, thus leading to an eventual predomination of dark energy which in turn drives the expansion.

See also

• Expanding universe
• Red shift
• Another theory

External links

• A series on ScienceBlogs [3]; [4]; [5]; [6]; [7]; [8] is pretty good