Somewhere around 13.8 billion years ago, the Universe began with a bang. In less than a second, the four fundamental forces -- electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction -- which initially were joined as a single even more fundamental force, separated. Suddenly, the Universe started to expand at an exponential rate. Cosmic inflation had begun. A little later, but still within this initial second, tiny particles called hadrons formed, and neutrinos ceased to interact with other particles. For the next ten seconds, particles called leptons dominated. Their short-lived rule gave way to photons, which governed for approximately 380,000 years. Newly formed atoms of hydrogen and helium took over next, but it wasn't until around 559 million years later that stars began to shine by fusing them together.

The Big Bang is the best theory we have to explain the birth and existence of the Universe. As astrophysicist Ethan Siegel wrote in his recent book Beyond the Galaxy:

"To this very day, there is no other model that is both consistent with General Relativity and explains the Hubble expansion of the Universe, the abundances of the light elements and the existence and properties of the cosmic microwave background; the Big Bang is the only one."

But while satisfying and substantially supported by the weight of scientific evidence, the defining theory of cosmology is not perfect. There remain three key problems.

The first is the Horizon Problem. If we look far out into space, billions of light years away, we see photons with the same temperature -- roughly 2.725 degrees Kelvin. If we look in another direction, we find the same thing. What a coincidence! In fact, when astronomers look in all directions, no matter how distant, they find that all regions have the same temperature. This is incredibly puzzling, Siegel says, "since these regions are separated by distances that are greater than any signal, even light, could have traveled in the time since the Universe was born." The Big Bang offers no explanation for this fascinating quirk.

Yet another quirk unexplained by the Big Bang is the Flatness Problem. Almost all the evidence collected by cosmologists indicates that the Universe is flat. Like a sheet of paper on a desk, spacetime shows almost no curvature whatsoever. Within the context of the Big Bang, this seems extremely unlikely.

Lastly, we arrive at the Monopole Problem. The immense energies produced by the Big Bang should have created a magnetic particle that breaks the mold. All magnets have two poles, a north and a south. Even when a magnet is snapped in half the two poles remain. But this particle would effectively be a magnet with only one pole: a magnetic monopole!

In 2014, researchers created a synthetic magnetic monopole in the laboratory, but physicists have yet to find one of these particles in nature. Blas Cabrera, a researcher at Stanford and the current leader of the Cryogenic Dark Matter Search experiment, detected a candidate monopole back in 1982, but no other experiments have replicated his results.

So what do we do with these three puzzling problems? Do we simply ignore them?

"It is tempting to look past these three problems as not problems at all, but rather as simply the conditions that the Universe started off with..." Siegel writes. "The Big Bang has enough successes that it is easy enough to sweep these problems under the rug and not be bothered by them."

"But that would be a terrible, non-scientific attitude to take... As soon as we convince ourselves that something is a question that science cannot answer, it becomes a self-fulfilling prophecy."

Scientists do not turn away from problems because they are difficult. To the contrary, scientists tackle problems precisely because they are difficult! To the discoverer goes the notoriety, and to the world goes something far more important: knowledge.

Primary Source: Ethan Siegel. Beyond the Galaxy: How Humanity Looked Beyond Our Milky Way and Discovered the Entire Universe. 2015. World Scientific.

(Images: AP Photo/Elise Amendola, NASA / WMAP Science Team, Sbyrnes321)