Posts tagged multiverse
Posts tagged multiverse
25 July 2011 by Marcus Chown
IT HAS been called the Goldilocks paradox. If the strong nuclear force which glues atomic nuclei together were only a few per cent stronger than it is, stars like the sun would exhaust their hydrogen fuel in less than a second. Our sun would have exploded long ago and there would be no life on Earth. If the weak nuclear force were a few per cent weaker, the heavy elements that make up most of our world wouldn’t be here, and neither would you.
If gravity were a little weaker than it is, it would never have been able to crush the core of the sun sufficiently to ignite the nuclear reactions that create sunlight; a little stronger and, again, the sun would have burned all of its fuel billions of years ago. Once again, we could never have arisen.
Such instances of the fine-tuning of the laws of physics seem to abound. Many of the essential parameters of nature - the strengths of fundamental forces and the masses of fundamental particles - seem fixed at values that are “just right” for life to emerge. A whisker either way and we would not be here. It is as if the universe was made for us.
What are we to make of this? One possibility is that the universe was fine-tuned by a supreme being - God. Although many people like this explanation, scientists see no evidence that a supernatural entity is orchestrating the cosmos. The known laws of physics can explain the existence of the universe that we observe. To paraphrase astronomer Pierre-Simon Laplace when asked by Napoleon why his book Mécanique Céleste did not mention the creator: we have no need of that hypothesis.
Another possibility is that it simply couldn’t be any other way. We find ourselves in a universe ruled by laws compatible with life because, well, how could we not?
This could seem to imply that our existence is an incredible slice of luck - of all the universes that could have existed, we got one capable of supporting intelligent life. But most physicists don’t see it that way.
The most likely explanation for fine-tuning is possibly even more mind-expanding: that our universe is merely one of a vast ensemble of universes, each with different laws of physics. We find ourselves in one with laws suitable for life because, again, how could it be any other way?
The multiverse idea is not without theoretical backing. String theory, our best attempt yet at a theory of everything, predicts at least 10500 universes, each with different laws of physics. To put that number into perspective, there are an estimated 1025 grains of sand in the Sahara desert.
12:14 04 September 2006 by Stephen Battersby
Cosmology is one of the most creative and bizarre areas of science. Explore some of the strangest ideas in this exclusive feature
Could our universe be a membrane floating in higher dimensional space, repeatedly smashing into a neighbouring universe? According to an offshoot of string theory called braneworld, there are large extra dimensions of space, and while gravity can reach out into them, we are confined to our own “brane” universe with only three dimensions. Neil Turok of Cambridge University in the UK and Paul Steinhardt of Princeton University in New Jersey, US, have worked out how the big bang could have been sparked when our universe clashed violently with another. These clashes repeat, producing a new big bang every now and then - so if the cyclic universe model is right, the cosmos could be immortal.
When matter is compressed to extreme densities at the centre of a black hole, it might bounce back and create a new baby universe. The laws of physics in the offspring might differ slightly, and at random, from the parent - so universes might evolve, suggests Lee Smolin of the Perimeter Institute in Waterloo, Canada. Universes that make a lot of black holes have a lot of children, so eventually they come to dominate the population of the multiverse. If we live in a typical universe, then it ought to have physical laws and constants that optimise the production of black holes. It is not yet known whether our universe fits the bill.
One of the most outlandish new theories of cosmology is that space-time is actually a superfluid substance, flowing with zero friction. Then if the universe is rotating, superfluid spacetime would be scattered with vortices, according to physicists Pawel Mazur of the University of South Carolina and George Chapline at Lawrence Livermore lab in California - and those vortices might have seeded structures such as galaxies. Mazur suggests that our universe might have been born in a collapsing star, where the combination of stellar matter and superfluid space could spawn dark energy, the repulsive force that is accelerating the expansion of the universe.
Why does the universe have properties that are “just right” to permit the emergence of life? Tinker with a few physical constants and we would end up with no stars, or no matter, or a universe that lasts only for the blink of an eye. One answer is the anthropic principle: the universe we see has to be hospitable, or we would not be here to observe it. Recently the idea has gained some strength, because the theory of inflation suggests that there may be an infinity of universes out there, and string theory hints that they might have an almost infinite range of different properties and physical laws. But many cosmologists dismiss the anthropic principle as being non-science, because it makes no testable predictions.
Dark matter might not really be “stuff” - it could just be a misleading name for the odd behaviour of gravity. The theory called MOND (modified Newtonian dynamics), suggests that gravity does not fade away as quickly as current theories predict. This stronger gravity can fill the role of dark matter, holding together galaxies and clusters that would otherwise fly apart. A new formulation of MOND, consistent with relativity, has rekindled interest in the idea, although it may not fit the pattern of spots in the cosmic microwave background.
Three mysteries of modern cosmology could be wrapped up in one ghostly presence. After making an adjustment to Einstein’s general theory of relativity, a team of physicists found a strange substance popping out of their new theory, the “ghost condensate”. It can produce repulsive gravity to drive cosmic inflation in the big bang, while later on it could generate the more sedate acceleration that is ascribed to dark energy. Moreover, if this slippery substance clumps together, it could form dark matter.
The pattern of spots in the cosmic microwave background has a suspicious deficiency: there are surprisingly few big spots. One possible explanation is that the universe is small - so small that, back when the microwave background was being produced, it just could not hold those big blobs. If so, space would have to wrap around on itself somehow. Possibly the oddest suggestion is that the universe is funnel-shaped, with one narrow end and one flared end like the bell of a trumpet. The bent-back curvature of space in this model would also stretch out any smaller microwave spots from round blobs into the little ellipses that are indeed observed.
Why do opposite sides of the universe look the same? It’s a puzzle because the extremes of today’s visible universe should never have been in touch. Even back in the early moments of the big bang, when these areas were much closer together, there wasn’t enough time for light - or anything else - to travel from one to another. There was no time for temperature and density to get evened out; and yet they are even. One solution: light used to move much faster. But to make that work could mean a radical overhaul of Einstein’s theory of relativity.
Dark matter might be made of the most elusive particles ever imagined - sterile neutrinos. They are hypothetical heavier cousins of ordinary neutrinos and would interact with other matter only through the force of gravity - making them essentially impossible to detect. But they might have the right properties to be “warm” dark matter, buzzing about at speeds of a few kilometres per second, forming the largish dark matter clumps mapped by recent observations. Sterile neutrinos could also help stars and black holes to form in the early universe, and give the kicks that send neutron stars speeding around our galaxy.
Maybe our universe isn’t real. Philosopher Nick Bostrom has claimed that we are probably living inside a computer simulation. Assuming it ever becomes possible to simulate consciousness, then presumably future civilisations would try it, probably many times over. Most perceived universes would be simulated ones - so chances are we are in one of them. In that case, perhaps all those cosmological oddities such as dark matter and dark energy are simply patches, stuck on to cover up early inconsistencies in our simulation.
String theory is one of the most famous ideas in modern physics, but it is also one of the most confusing.
At its heart is the idea that the fundamental particles we observe are not point-like dots, but rather tiny strings that are so small that our best instruments cannot tell that they are not points.
It also predicts that there are extra dimensions to space beyond the obvious length, breadth and depth, but we do not experience them because they are bunched up in tiny spaces.
While these notions are deeply strange, the key issue for string theorists has actually been the difficulty of testing their ideas.
This week, we met up with Edward Witten, one of the leading proponents of string theory (see Inside the tangled world of string theory). To accompany this interview, we’ve put together a beginner’s guide to one of physics’ strangest imaginative feats.
By Ron Cowen
Most cosmologists trace the birth of the universe to the Big Bang 13.7 billion years ago. But a new analysis of the relic radiation generated by that explosive event suggests the universe got its start eons earlier and has cycled through myriad episodes of birth and death, with the Big Bang merely the most recent in a series of starting guns.
That startling notion, proposed by theoretical physicist Roger Penrose of the University of Oxford in England and Vahe Gurzadyan of the Yerevan Physics Institute and Yerevan State University in Armenia, goes against the standard theory of cosmology known as inflation.
The researchers base their findings on circular patterns they discovered in the cosmic microwave background, the ubiquitous microwave glow left over from the Big Bang. The circular features indicate that the cosmos itself circles through epochs of endings and beginnings, Penrose and Gurzadyan assert. The researchers describe their controversial findings in an article posted at arXiv.org on November 17.
The circular features are regions where tiny temperature variations in the otherwise uniform microwave background are smaller than average. Those features, Penrose said, cannot be explained by the highly successful inflation theory, which posits that the infant cosmos underwent an enormous growth spurt, ballooning from something on the scale of an atom to the size of a grapefruit during the universe’s first tiny fraction of a second. Inflation would either erase such patterns or could not easily generate them.
“The existence of large-scale coherent features in the microwave background of this form would appear to contradict the inflationary model and would be a very distinctive signature of Penrose’s model” of a cyclic universe, comments cosmologist David Spergel of Princeton University. But, he adds, “The paper does not provide enough detail about the analysis to assess the reality of these circles.”
Penrose interprets the circles as providing a look back, past the glass wall of the most recent Big Bang, into the universe’s previous episode, or “aeon,” as he calls it. The circles, he suggests, were generated by collisions between supermassive black holes that occurred during this earlier aeon. The colliding black holes would have created a cacophony of gravitational waves — ripples in spacetime due to the acceleration of the giant masses. Those waves would have been spherical and uniformly distributed.
According to the detailed mathematics worked out by Penrose, when the uniform distribution of gravitational waves from the previous aeon entered the current aeon, they were converted into a pulse of energy. The pulse provided a uniform kick to the allotment of dark matter, the invisible material that accounts for more than 80 percent of the mass of the cosmos.
“The dark matter material along the burst therefore has this uniform character,” says Penrose. “This is what is seen as a circle in our cosmic microwave background sky, and it should look like a fairly uniform circle.”
Each circle has a lower-than-average variation in temperature, which is just what he and Gurzadyan found when they analyzed data from NASA’s orbiting Wilkinson Microwave Anisotropy Probe, or WMAP, which scanned the entire sky for nine years, and the balloon-borne BOOMERANG experiment, which studied microwave background over a smaller fraction of the heavens.
Because the team found similar circular features with two different detectors, Penrose says it’s unlikely he and his colleagues are being fooled by instrumental noise or other artifacts.
But Spergel says he is concerned that the team has not accounted for variations in the noise level of WMAP data acquired over different parts of the sky. WMAP examined different sky regions for different amounts of time. Maps of the microwave background generated from those regions studied the longest would have lower noise and smaller recorded variations in the temperature of the microwave glow. Those lower-noise maps could artificially produce the circles that Penrose and Gurzadyan ascribe to their model of a cyclic universe, Spergel says.
A new, more detailed map of the cosmic microwave background, now being conducted by the European Space Agency’s Planck mission, could provide a more definitive test of the theory, Penrose says.