Sixty years ago there was a rip-roaring debate over whether the universe actually had a beginning.
Was there really a creation moment dubbed the "Big Bang," where the universe spontaneously arose out of nothing - not even from ashes like the legendary Phoenix?
Or was the universe eternal, as Einstein himself imagined and worked into his concept of a cosmological constant that kept the universe forever in equilibrium?
It looks like cosmologists today might be able to have their cake, and eat it too.
An emerging new idea is that the universe had a Big Bang, but only one of countless creation moments in a "megaverse" that is eternally inflating with multiple big bangs, like a string of firecrackers going off.
In the 1950s, British astronomer Fred Hoyle was a strong proponent of the "steady state" universe. This theory held that universe is eternal; it always looked the way it does today. Stars are born and die endlessly inside majestic galaxies.
Yes, Hoyle had to concede the universe was expanding, as discovered by Edwin Hubble. But he said that space just spread on forever carrying galaxies with it like an infinitely long conveyor belt. New stars and galaxies were always replenishing the conveyor belt.
The steady state hypothesis vaporized away when the cosmic microwave background (CMB) radiation was accidentally discovered in the mid-1960s. This was the much-hypothesized afterglow as predicted by Big Bang theory. Primeval photons, attenuated by the stretching of space, were still hanging around as Big Bang fossils. Steady state theory cannot any account for this relic radiation.
But now we have the "eternal universe inflation" scenario. It predicts that our observable universe resides inside a single bubble embedded in a vast inflating multiverse of an infinity of other bubbles that are their own unique universes with their own unique set of physical constants. (This would help explain why dark energy is so weak – it is just one flavor of dark energy in the megaverse.)
These parallel universes are popping in and out of existence and colliding all the time, like popping kernels in a popcorn kettle. However, space between the bubble universes is rapidly expanding. This imposes a cosmic quarantine where the universes are out of reach of one another.
The notion of parallel universes is not new. It goes back to the Greek philosophers. But without any way of testing for them, parallel universes remain as illusory as the number of angels that can dance on the head of a pin.
But cosmologists are conducting observational tests of eternal inflation. The idea is that our universe should bump into a parallel universe. This would leave a telltale smudge on the cosmic microwave background.
This would happen if there were simultaneous big bangs occurring close together. The baby universes would bump into each other, but the space between the bubbles inflates faster than the speed of light, and so they quickly scatter apart.
The CMB is being mapped by several spaces telescopes, so a snapshot of how the universe looked 380,000 years after the Big Bang is emerging. It is mottled with slight ripples of temperature variations, which were the seeds for the construction of great filaments of galaxies.
A team lead by Stephen Feeney of the University College London ran a statistical program that looked for circular patterns on the CMB from seven-year's worth of data from NASA's WMAP satellite. Their program found four particular areas that look suspiciously like the pattern predicted from inflating megaverse theory. But is this just a cosmological Rorschach? The analysis at present is inconclusive.
This is not without precedent.
A particularly large area on the CMB, dubbed the "Cold Spot" has also attracted attention as possibly being too large and too cold to be easily explained (it also identified as one of Feeney's candidates). It has been speculated to be a super-void in the universe, or a hicky from a parallel universe, or just a statistical peculiarity.
The European Space Agency's Planck satellite is now mapping the CMB in higher resolution than ever. When the data are fully released in 2013 they can be used to retest Feeney's statistical models.
I think that Fred Hoyle, who passed away in 2001, might have felt vindicated by this new line of investigation.
Even if everyone agrees these features are caused by shadow universes, we still could not deduce anything about them aside from their ghostly thumbprint.
Image credits: S. Feeney, NASA