What was prior to the big bang

As in the real universe, density differences between regions form a bell curve around zero. If this possible solution does indeed dominate the wave function for minisuperspace, it becomes plausible to imagine that a far more detailed and exact version of the no-boundary wave function might serve as a viable cosmological model of the real universe.

The other potentially dominant universe shape is nothing like reality. As it widens, the energy infusing it varies more and more extremely, creating enormous density differences from one place to the next that gravity steadily worsens. Density variations form an inverted bell curve, where differences between regions approach not zero, but infinity.

If this is the dominant term in the no-boundary wave function for minisuperspace, then the Hartle-Hawking proposal would seem to be wrong. The two dominant expansion histories present a choice in how the path integral should be done. If the dominant histories are two locations on a map, megacities in the realm of all possible quantum mechanical universes, the question is which path we should take through the terrain.

Researchers have forked down different paths. In their paper, Turok, Feldbrugge and Lehners took a path through the garden of possible expansion histories that led to the second dominant solution. Lacking a causal element, lapse is not quite our usual notion of time. Yet Turok and colleagues argue partly on the grounds of causality that only real values of lapse make physical sense. And summing over universes with real values of lapse leads to the wildly fluctuating, physically nonsensical solution.

He and Hartle analyzed the issue of the contour of integration in He and his colleagues argue that, in the minisuperspace case, only contours that pick up the good expansion history make sense. Imaginary numbers pervade quantum mechanics. To team Hartle-Hawking, the critics are invoking a false notion of causality in demanding that lapse be real. According to Hertog, Hawking seldom mentioned the path integral formulation of the no-boundary wave function in his later years, partly because of the ambiguity around the choice of contour.

He regarded the normalizable expansion history, which the path integral had merely helped uncover, as the solution to a more fundamental equation about the universe posed in the s by the physicists John Wheeler and Bryce DeWitt. Wheeler and DeWitt — after mulling over the issue during a layover at Raleigh-Durham International — argued that the wave function of the universe, whatever it is, cannot depend on time, since there is no external clock by which to measure it.

And thus the amount of energy in the universe, when you add up the positive and negative contributions of matter and gravity, must stay at zero forever. The no-boundary wave function satisfies the Wheeler-DeWitt equation for minisuperspace.

In the final years of his life, to better understand the wave function more generally, Hawking and his collaborators started applying holography — a blockbuster new approach that treats space-time as a hologram. Hawking sought a holographic description of a shuttlecock-shaped universe, in which the geometry of the entire past would project off of the present. But Turok sees this shift in emphasis as changing the rules.

The universe didn't expand into space; space itself expanded. No one knows exactly what was happening in the universe until 1 second after the Big Bang, when the universe cooled off enough for protons and neutrons to collide and stick together. Many scientists do think that the universe went through a process of exponential expansion called inflation during that first second.

This would have smoothed out the fabric of space-time and could explain why matter is so evenly distributed in the universe today. It's possible that before the Big Bang, the universe was an infinite stretch of an ultrahot, dense material, persisting in a steady state until, for some reason, the Big Bang occured. This extra-dense universe may have been governed by quantum mechanics, the physics of the extremely small scale, Carroll said.

The Big Bang, then, would have represented the moment that classical physics took over as the major driver of the universe's evolution. For Stephen Hawking, this moment was all that mattered: Before the Big Bang, he said, events are unmeasurable, and thus undefined. Or perhaps there was something else before the Big Bang that's worth pondering. One idea is that the Big Bang isn't the beginning of time, but rather that it was a moment of symmetry. In this idea, prior to the Big Bang, there was another universe, identical to this one but with entropy increasing toward the past instead of toward the future.

Increasing entropy, or increasing disorder in a system, is essentially the arrow of time, Carroll said, so in this mirror universe , time would run opposite to time in the modern universe and our universe would be in the past.

Proponents of this theory also suggest that other properties of the universe would be flip-flopped in this mirror universe. Galaxies move away from one another because they are literally carried by the stretch of space itself. Like an elastic fabric, space stretches out and the galaxies are carried along, like corks floating down a river.

So, galaxies are not like pieces of shrapnel flying away from a central explosion. There is no central explosion. The universe expands in all directions and is perfectly democratic: every point is equally important. Someone in a faraway galaxy would see other galaxies moving away just like we do. Playing the cosmic movie backward, we see matter getting squeezed more and more into a shrinking volume of space.

Temperature rises, pressure rises, things break apart. Molecules get broken down into atoms, atoms into nuclei and electrons, atomic nuclei into protons and neutrons, and then protons and neutrons into their constituent quarks.

For example, hydrogen atoms dissociate at about , years after the Big Bang, atomic nuclei at about one minute, and protons and neutrons at about one-hundredth of a second.

How do we know? We have found the radiation left over from when the first atoms formed the cosmic microwave background radiation and discovered how the first light atomic nuclei were made when the universe was merely a few minutes old. These are the cosmic fossils that show us the way backward. Currently, our experiments can simulate conditions that happened when the universe was roughly one trillionth of a second old.

When talking about the early universe, we must let go of our human standards and intuitions of time. But eventually we hit a wall of ignorance, and all we can do is extrapolate our current theories, hoping that they will give us some hints of what was going on much earlier, at energies and temperatures we cannot test in the lab.

This is the realm of quantum mechanics, where distances are so tiny that we must rethink space not as a continuous sheet but as a granular environment.