Excerpt from reddit:
Today it was announced that the BICEP2 cosmic microwave background telescope at the south pole has detected the first evidence of gravitational waves caused by cosmic inflation.
This is one of the biggest discoveries in physics and cosmology in decades, providing direct information on the state of the universe when it was only 1e-34 seconds old, and energy scales near the Planck energy, as well as confirmation of the existence of gravitational waves.
Quick run down for those not in the field: The BICEP telescope measures the polarization of the Cosmic Microwave Background (CMB).
The CMB is light that was released ~380,000 years after the Big Bang. The Universe was a hot dense plasma right after the Big Bang. As it expanded and cooled, particles begin to form and be stable. Stable protons and electrons appear, but because the Universe was so hot and so densely packed, they couldn't bind together to form stable neutral hydrogen, before a high-energy photon came zipping along and smashed them apart. As the Universe continued to expand and cool, it eventually reached a temperature cool enough to allow the protons and the electrons to bind. This binding causes the photons in the Universe that were colliding with the formerly charged particles to stream freely throughout the Universe. The light was T ~= 3000 Kelvin then. Today, due to the expansion of the Universe, we measure it's energy to be 2.7 K.
Classical Big Bang cosmology has a few open problems, one of which is the Horizon problem. The Horizon problem states that given the calculated age of the Universe, we don't expect to see the level of uniformity of the CMB that we measure. Everywhere you look, in the microwave regime, through out the entire sky, the light has all the same average temperature/energy, 2.725 K. The light all having the same energy suggests that it it was all at once in causal contact. We calculate the age of the Universe to be about 13.8 Billion years. If we wind back classical expansion of the Universe we see today, we get a Universe that is causally connected only on ~ degree sized circles on the sky, not EVERYWHERE on the sky. This suggests either we've measured the age of the Universe incorrectly, or that the expansion wasn't always linear and relatively slow like we see today.
One of the other problems is the Flatness Problem. The Flatness problem says that today, we measure the Universe to be geometrically very close to flatness, like 1/100th close to flat. Early on, when the Universe was much, much smaller, it must've been even CLOSER to flatness, like 1/10000000000th. We don't like numbers in nature that have to be fine-tuned to a 0.00000000001 accuracy. This screams "Missing physics" to us.
Another open problem in Big Bang cosmology is the magnetic monopole/exotica problem. Theories of Super Symmetry suggest that exotic particles like magnetic monopoles would be produced in the Early Universe at a rate of like 1 per Hubble Volume. But a Hubble Volume back in the early universe was REALLY SMALL, so today we would measure LOTS of them, but we see none.
One neat and tidy way to solve ALL THREE of these problems is to introduce a period of rapid, exponential expansion, early on in the Universe. We call this "Inflation". Inflation would have to blow the Universe up from a very tiny size about e60 times, to make the entire CMB sky that we measure causally connected. It would also turn any curvature that existed in the early Universe and super rapidly expand the radius of curvature, making everything look geometrically flat. It would ALSO wash out any primordial density of exotic particles, because all of a sudden space is now e60 times bigger than it is now.
This sudden, powerful expansion of space would produce a stochastic gravitational wave background in the Universe. These gravitational waves would distort the patterns we see in the CMB. These CMB distortions are what BICEP and a whole class of current and future experiments are trying to measure.
Discuss. Religious sceptics welcome too.