gravitational waves were first theorized by einstein; though difficult to detect? - they're still there... ;)
The universe, as we all know, began with a bang, but how did the aftershock shape reality, exactly? One possible option is inflation theory, which posits that space-time underwent an extremely rapid period of expansion in the first moments after the Big Bang. Inflation theory experts and skeptics weighed the evidence in “Ripples From The Big Bang: Listening To The Beginning of Time,” part of the Big Ideas series at the 2014 World Science Festival.
“To understand the origin of the universe, you need to have an understanding of that force which is relevant over large distances and cosmic scales: That is the force of gravity,” panel moderator and World Science Festival co-founder Brian Greene said.
Scientists’ understanding of gravity has evolved greatly from the time of Isaac Newton. Albert Einstein deduced that space itself is the medium for transmitting the force of gravity; planets fall into orbits around stars because the fabric of space curves, trapping them in a valley. In Einstein’s conception, gravity bent light rays; Newton had merely suggested this might be possible in his book “Optics,” but did not pursue the matter in depth. Einstein was validated in 1919, when astronomers observed the stars during a total eclipse. During that event, the light from the Hyades star cluster was warped by its passage close to the sun’s gravitational field, but the stars would still be visible thanks to the darkness of the eclipse. Astronomers found that the Hyades seemed to slightly shift its position as the light was bent around the sun, thus providing the first major confirmation of general relativity.
Later insights into gravity and its place in the universe followed: Edwin Hubble found that the galaxies all seem to be rushing away from us, which led to the insight that the universe was expanding. If you run the clock backwards, than the universe must have been closer and closer together until it was compressed into a single point. But what was pushing everything apart?
One of the best windows that researchers have into the Big Bang is the cosmic microwave background radiation, the heat lingering beneath all radio telescope readings and unassociated with any particular star or galaxy. One particular feature of the background radiation puzzled scientists for years: the fact that the temperature and curvature of space appears to be pretty much the same, even though they’re much too far apart to have received signals transmitted by light. In the 1970s, researcher Alan Guth proposed that a tremendously exponential period of expansion could solve this and other lingering puzzles arising from the Big Bang. But is there a way to prove inflation? There might be, by closely studying information hidden in the cosmic background radiation.
One of the predicted effects of inflation is the creation of gravitational waves, or ripples in space-time. And it so happens there is a possible way to detect the signature of these ripples in the cosmic microwave background radiation. “As a gravitational wave passes through early universe, it has the potential to distort space-time… and can impart a particular polarization to light,” panelist and Harvard researcher John Kovac said. And later, the geometry of how space-time stretched “can impart a particular swirling pattern to polarization of light.”
In March, Kovac’s team, working with the BICEP2 telescope on the South Pole, dropped a bombshell on the scientific community: In their analysis of the cosmic background radiation, they’d found what looked an awful lot like that swirling pattern of polarization.
The BICEP2 discovery “is very strong confirmation that inflation took place,” Guth said. “It also allows us to detect, for first time, the rate of expansion of the universe at the time of inflation, which is a very important number that we otherwise don’t know.”
Princeton University physicist Paul Steinhardt, however, doesn’t think that scientists should jump to the conclusion that the pattern stems from inflation: “There are various possible sources of gravitational waves, including things we might not yet have thought of.”
One problem with trying to develop cosmological theories is that “we only have one universe to observe,” panelist and Columbia physicist Amber Miller noted.
Steinhardt still emphasized viewing the initial BICEP2 results with heavy skepticism.
“We really want to make sure this signal we’re seeing is something that’s really happening in the most distant parts of the universe, and not due to the effects of lensing of galaxies in the foreground, or dust in our own galaxy, or effects in the atmosphere, or even effects in the telescope,” Steinhardt said. “Any one of those things can take a pattern that has no swirls and turn them into a swirly pattern.”
Kovac said his team was fairly confident that the pattern was not due to random chance, nor telescope or atmospheric conditions. The possible impact of galactic dust was a little more uncertain, he admitted. But, observations taken by the Planck satellite may help in this arena, by allowing researchers to analyze the BICEP2 signal with more accurate projections of dust in our galactic region in mind.
Steinhardt’s skepticism also stretches to inflation theory itself. “The major concern about inflation theory to me is its extraordinary flexibility in terms of what it can produce,” he said.
Inflation theory, he said, has three areas with a lot of flexibility. It is extremely sensitive to the initial conditions of the Big Bang; you can adjust the curve of the inflationary field to change your predictions at will. And most models of inflation theory predict a multiverse, rather than a uniform universe; you can account for every conceivable cosmological possibility.
“There’s no test you can make of the theory that can disprove it. And if a theory gets to a point where you cannot conceive of a test to rule it out, that crosses a line,” Steinhardt said. “From my point of view that takes you past science into what you might call metaphysics.”
But Guth disagreed. “Science is an arena of competing ideas, and right now inflation is by far the most widespread idea in cosmology. It is true it is not a unique theory but a class of theories, but what we’ve found so far is the simplest versions of inflation are the ones that fit the data beautifully.”
Whether Planck’s results can support the BICEP2 team’s finding remains to be seen—some early analyses released just after the talk are not encouraging. But scientists will continue to push forward.
“It’s just astounding that we can sit here and seriously talk about what happened a trillionth of a trillionth of a trillionth of a second after the beginning,” Greene said. “We can talk about it observationally, theoretically…it truly is the golden age of cosmology.”
Want to see the full “Ripples From The Big Bang” program? Check out the livestream video here.
Photo credit: Greg Kessler
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