A Universe is a set of photons that interact together, and which do NOT interact, generally, with photons of other universes which have other characteristics. This might better be said to be the spason of the universe that interact determine the characteristics of the universe, but since they determine the properties of the photons. This then amounts to the same thing.Theoretical physics has brought us the notion that our single universe is not necessarily the only game in town. Satellite data from WMAP, along with string theory and its 11- dimensional hyperspace idea has produced the concept of the multiverse, where the Big Bang could have produced many different universes instead of a single uniform universe. The idea has gained popularity recently, so it was only a matter of time until someone asked the question of how many multiverses could possibly exist. The number, according to two physicists, could be â€œhumongous.â€
Andrei Linde and Vitaly Vanchurin at Stanford University in California, did a few back-of- the- envelope calculations, starting with the idea that the Big Bang was essentially a quantum process which generated quantum fluctuations in the state of the early universe. The universe then underwent a period of rapid growth called inflation during which these perturbations were â€œfrozen,â€ creating different initial classical conditions in different parts of the cosmos. Since each of these regions would have a different set of laws of low energyphysics, they can be thought of as different universes.
Linde and Vanchurin then estimated how many different universes could have appeared as a result of this effect. Their answer is that this number must be proportional to the effect that caused the perturbations in the first place, a process called slow roll inflation, â€” the solution Linde came up with previously to answer the problem of the bubbles of universes colliding in the early inflation period. In this model, inflation occurred from a scalar field rolling down a potential energy hill. When the field rolls very slowly compared to the expansion of the universe, inflation occurs and collisions end up being rare.
Using all of this (and more â€“ see their paper here) Linde and Vanchurin calculate that the number of universes in the multiverse and could be at least 10^10^10^7, a number which is definitely â€œhumungous,â€ as they described it.
The next question, then, is how many universes could we actually see? Linde and Vanchurin say they had to invoke the Bekenstein limit, where the properties of the observer become an important factor because of a limit to the amount of information that can be contained within any given volume of space, and by the limits of the human brain.
The total amount of information that can be absorbed by one individual during a lifetime is about 10^16 bits. So a typical human brain can have 10^10^16 configurations and so could never distinguish more than that number of different universes.
Given some of scienceâ€™s current ideas about high-energy physics, it is plausible that those other universes might each have different physical interactions. So perhaps itâ€™s no mystery that we would happen to occupy the rare universe in which conditions are just right to make life possible. This is analogous to how, out of the many planets in our universe, we occupy the rare one where conditions are right for organic evolution.
The possibility of a multiverse comes from both string theory and inflation theory, the idea that our universe underwent a rapid expansion just after the Big Bang. Inflation theory does a good job of explaining why space is fairly smooth on large scales, but researchers canâ€™t explain what started the expansion and what stopped it. These problems have led physicists to consider the possibility that inflation could occur at other places and times, generating new universes in addition to our own.
The idea of a multiverse is highly controversial. One problem is metaphysical: the universe seems big already, without having to contend with a potentially infinite number of others. Yet perhaps a bigger problem is scientific. If observations are limited to our own observable universe, how can scientists test whether a bigger multiverse exists? The answer to that has been that, from time to time, another universe in the multiverse might collide through ours, leaving a â€œwakeâ€ in its path. But figuring out precisely what such a wake would look like hasnâ€™t been easy.
Now, however, Kris Sigurdson of the University of British Columbia in Vancouver and others say they have calculated the detailed features of a cosmic wake. They have considered the possibility that our universe collided with another before our inflation period, because, they say, the latter would have erased the wakeâ€™s evidence. Even though this happened more than 13 billion years ago, the wake would have been preserved in the cosmic microwave background (CMB), which was formed some 380,000 years into the universeâ€™s existence.
The focus of the prediction is in the polarization of photons in the CMB. Photons have two transverse polarization states, and any that come from a certain region in the CMB might be mostly in the same polarization state, or in a mix of both. Sigurdson and colleagues calculate that, providing the wake was big enough, it ought to imprint the CMB with a characteristic â€œdouble peakâ€: two close rings where the photons sway towards a single polarization state.
The prediction is not strictly the first to arise from multiverse theory. In 2007 researchers at the University of California at Santa Cruz, US, also suggested that a cosmic wake could imprint itself on the CMB; then, earlier this year, a group led by Hiranya Peiris of University College London found hints that this prediction was true. But these predicted features were too vague, say Sigurdson and colleagues, and might have existed in the CMB anyway.
Evidence for string theory?
â€œ[Our] features represent the first verifiable prediction of the multiverse paradigm,â€ write Sigurdson and colleagues in their preprint, which they uploaded to the arXiv server last month. â€œA detection of a bubble collision would confirm the existence of the multiverse, provide compelling evidence for the string theory landscape, and sharpen out picture of the universe and its origins.â€Physics World was unable to speak to the researchers about their preprint because they are submitting it to a journal that employs an embargo policy.
If the prediction is correct, it should be possible to test it in upcoming data from the European Space Agencyâ€™s Planck space observatory and future CMB missions, say the researchers. Yet Bennett, the principal investigator on NASAâ€™s Wilkinson Microwave Anisotropy Probe, another CMB space observatory, thinks the detection of a cosmic wake would nonetheless be â€œextremely unlikelyâ€. He says the amplitude of a wake would have to be just right: too small and we wouldnâ€™t see it; too big and it would probably have had severe consequences for our universeâ€™s structure. The number of collisions would also have to be â€œfine-tunedâ€, he says.