Has the Big Bang Theory been proven?

By ?Andrew May and?Elizabeth Howell

The Big Bang Theory is the leading explanation for how?the universe?began. Simply put, it says the universe as we know it started with an infinitely hot and dense single point that inflated and stretched — first at unimaginable speeds, and then at a more measurable rate — over the next 13.7 billion years to the still-expanding cosmos that we know today.

Existing technology doesn't yet allow astronomers to literally peer back at the universe's birth, much of what we understand about the Big Bang comes from mathematical formulas and models. Astronomers can, however, see the "echo" of the expansion through a phenomenon known as the?cosmic microwave background.

While the majority of the astronomical community accepts the theory, there are some theorists who have?alternative explanations besides the Big Bang?— such as eternal inflation or an oscillating universe.

THE BIG BANG: THE BIRTH OF THE UNIVERSE

Around 13.7 billion years ago, everything in the entire universe was condensed in an infinitesimally small singularity, a point of infinite denseness and heat.?

Suddenly, an explosive expansion began, ballooning our universe outwards faster than the?speed of light. This was a period of cosmic inflation that lasted mere fractions of a second — about 10^-32 of a second, according to physicist Alan Guth’s?1980 theory?that changed the way we think about the Big Bang forever.?

When cosmic inflation came to a sudden and still-mysterious end, the more classic descriptions of the Big Bang took hold. A flood of matter and radiation, known as "reheating," began populating our universe with the stuff we know today: particles, atoms, the stuff that would become stars and galaxies and so on.

This all happened within just the first second after the universe began, when the temperature of everything was still insanely hot, at about 10 billion degrees Fahrenheit (5.5 billion Celsius),?according to NASA. The cosmos now contained a vast array of fundamental particles such as neutrons, electrons and protons — the raw materials that would become the building blocks for everything that exists today.

This early "soup" would have been impossible to actually see because it couldn't hold visible light. "The free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds," NASA stated. Over time, however, these free electrons met up with nuclei and created neutral atoms or atoms with equal positive and negative electric charges.?

This allowed light to finally shine through, about 380,000 years after the Big Bang.

Sometimes called the "afterglow" of the Big Bang, this light is more properly known as the?cosmic microwave background?(CMB). It was first predicted by Ralph Alpher and other scientists in 1948 but?was found only by accident almost 20 years later.

Related:?Peering back to the Big Bang & early universe

This accidental discovery happened when Arno Penzias and Robert Wilson, both of Bell Telephone Laboratories in New Jersey, were building a radio receiver in 1965 and picked up higher-than-expected temperatures,?according to a NASA article. At first, they thought the anomaly was due to pigeons trying to roost inside the antenna and their waste, but they cleaned up the mess and?killed the pigeons?and the anomaly persisted.

Simultaneously, a Princeton University team led by Robert Dicke was trying to find evidence of the CMB and realized that Penzias and Wilson had stumbled upon it with their strange observations. The two groups each published papers in the Astrophysical Journal in 1965.

BIG BANG THEORY FAQS ANSWERED BY AN EXPERT

We asked Jason Steffens, assistant professor of physics and astronomy at the University of Nevada, Las Vegas, a few frequently asked questions about the Big Bang Theory.?

Has the Big Bang Theory been proven?

This isn't really a statement that we can make in general. The best we can do is say that there is strong evidence for the Big Bang Theory and that every test we throw at it comes back in support of the theory. Mathematicians prove things, but scientists can only say that the evidence supports a theory with some degree of confidence that is always less than 100%.

So, a short answer to a slightly different question is that all of the observational evidence that we've gathered is consistent with the predictions of the Big Bang Theory. The three most important observations are:

1)?The Hubble Law shows that distant objects are receding from us at a rate proportional to their distance — which occurs when there is uniform expansion in all directions. This implies a history where everything was closer together.

2)?The properties of the cosmic microwave background radiation (CMB). This shows that the universe went through a transition from an ionized gas (a plasma) and a neutral gas. Such a transition implies a hot, dense early universe that cooled as it expanded. This transition happened after about 400,000 years following the Big Bang.

3)?The relative abundances of light elements (He-4, He-3, Li-7, and Deuterium). These were formed during the era of Big Bang Nucleosynthesis (BBN) in the first few minutes after the Big Bang. Their abundances show that the universe was really hot and really dense in the past (as opposed to the conditions when the CMB was formed, which was just regular hot and dense — there's about a factor of a million difference in temperature between when BBN occurred and when the CMB occurred).

Is there any occurrence that contradicts the Big Bang Theory?

Not that I know of. There are some issues that arise with the simplest model of the Big Bang, but those can be resolved by invoking a physical process that is still consistent with the basic premise of the Big Bang Theory. Specifically, the fact that the CMB temperature is the same everywhere, that the universe does not appear to have any curvature, and that density fluctuations from quantum mechanical predictions do not produce galaxy clusters of the right size and shape today. These three issues are resolved with the theory of inflation — which is part of the broader Big Bang Theory.

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