Anthropic Arguments

NYTimes op-ed about science and faith:

Over the years I have often asked my physicist colleagues why the laws of physics are what they are. The answers vary from “that’s not a scientific question” to “nobody knows.” The favorite reply is, “There is no reason they are what they are — they just are.” The idea that the laws exist reasonlessly is deeply anti-rational. After all, the very essence of a scientific explanation of some phenomenon is that the world is ordered logically and that there are reasons things are as they are. If one traces these reasons all the way down to the bedrock of reality — the laws of physics — only to find that reason then deserts us, it makes a mockery of science.

Can the mighty edifice of physical order we perceive in the world about us ultimately be rooted in reasonless absurdity? If so, then nature is a fiendishly clever bit of trickery: meaninglessness and absurdity somehow masquerading as ingenious order and rationality.

Although scientists have long had an inclination to shrug aside such questions concerning the source of the laws of physics, the mood has now shifted considerably. Part of the reason is the growing acceptance that the emergence of life in the universe, and hence the existence of observers like ourselves, depends rather sensitively on the form of the laws. If the laws of physics were just any old ragbag of rules, life would almost certainly not exist.

A second reason that the laws of physics have now been brought within the scope of scientific inquiry is the realization that what we long regarded as absolute and universal laws might not be truly fundamental at all, but more like local bylaws. They could vary from place to place on a mega-cosmic scale. A God’s-eye view might reveal a vast patchwork quilt of universes, each with its own distinctive set of bylaws. In this “multiverse,” life will arise only in those patches with bio-friendly bylaws, so it is no surprise that we find ourselves in a Goldilocks universe — one that is just right for life. We have selected it by our very existence.

New Scientist article on entangled universes:

String theory's selling point had always been that it could make unique predictions about the properties of our universe. This made it more aesthetically pleasing than anthropic arguments, which say that certain aspects of the universe - like the constants that characterise the laws of physics - are the way they are because otherwise we wouldn't be here to wonder about them.

Yet string theory does not just describe one universe. It describes 10500 universes, each one a quantum vacuum with different physical properties. So why was ours the universe that grew large? "String theorists, who so much hoped to avoid the anthropic principle, have now been forced to invoke it to explain why our vacuum was selected out from the 10500 other string vacuums," says Mersini-Houghton.

Anthropic arguments leave many physicists queasy. They would prefer an explanation for the universe's properties that has nothing to do with our existence. Rather than abandon string theory completely, however, Mersini-Houghton was convinced there must be a way to thin down the forest of string vacuums without using the anthropic principle. She and her collaborator Richard Holman of Carnegie Mellon University in Pittsburgh, Pennsylvania, had a hunch that matter and gravity might have some kind of dynamic effect that whittles down the number of vacuums to a small bunch that eventually grows into our universe and its neighbours.

According to string theory, each possible universe has different conditions. If a patch of vacuum is to lead to a universe like ours, the important thing is that it must grow large. This means something must oppose gravity, which tends to suck together the mass-energy of the vacuum and shrink it.

I hate anthropic arguments because they somehow always end up being used to prove the existence of Dog and that we are somehow cosmically special.  And I'm perfectly comfortable with the unknown and the unknowable--striving for knowledge and understanding doesn't mean I expect to know or understand everything.  It might be cliched, but the journey is the key for me, not the destination...

ntodd

The void: Imprint of another universe?

    * 24 November 2007
    * From New Scientist Print Edition. Subscribe and get 4 free issues.
    * Marcus Chown

IN AUGUST, radio astronomers announced that they had found an enormous hole in the universe. Nearly a billion light years across, the void lies in the constellation Eridanus and has far fewer stars, gas and galaxies than usual. It is bigger than anyone imagined possible and is beyond the present understanding of cosmology. What could cause such a gaping hole? One team of physicists has a breathtaking explanation: "It is the unmistakable imprint of another universe beyond the edge of our own," says Laura Mersini-Houghton of the University of North Carolina at Chapel Hill.

It is a staggering claim. If Mersini-Houghton's team is right, the giant void is the first experimental evidence for another universe. It would also vindicate string theory, our most promising understanding of how the universe works at its most fundamental level. And it would do away with the anthropic arguments that have plagued string theorists in recent years because they say we are the reason the cosmos is the way it is. The hole in the universe is a big deal.

The giant void first showed up in maps of the afterglow of the big bang. In 2004, NASA's WMAP satellite made the most detailed measurements to date of the temperature of the cosmic background radiation. This microwave radiation gains a small amount of energy when it passes through a region of space populated by matter, making it appear slightly warmer in that direction. In contrast, radiation passing through an empty void loses energy, and so it appears cooler.

The WMAP team noticed an abnormally large cold spot where the temperature was between 20 and 45 per cent lower than the average for the rest of the sky, suggestive of a void. The spot covers a few degrees of the sky - many times more than the full moon. However, without knowing how far away the void was, it was difficult to tell its size.

Things began to change as researchers analysed the Sloan Digital Sky Survey, the largest 3D map of galaxies made so far. Once they knew how far away various galaxies were, they were able to calculate that the WMAP cold spot coincides with an enormous void that has grown to around 900 million light years across. Located about 8 billion years away, the void contains about 20 to 45 per cent fewer galaxies than you would expect.

This was confirmed in August by Lawrence Rudnick, Shea Brown and Liliya Williams of the University of Minnesota in Minneapolis, who were analysing a survey of radio-emitting galaxies carried out by the Very Large Array of telescopes at the National Radio Astronomy Observatory near Socorro, New Mexico.

A mere 5 per cent of the universe is filled with galaxy clusters, the other 95 per cent is mysterious voids. There are plenty of small voids, but the bigger they get the rarer they become. No one expected one 900 million light years across.

A void so big is virtually impossible to explain within standard cosmology. According to our best theories, the seeds of galaxy clusters and voids were sown shortly after the big bang, when the universe was a roiling vacuum of quantum fluctuations that were then magnified by a period of superfast expansion called inflation. Fluctuations of all sizes are possible, though larges ones are rare. "Any fluctuation leading to a void as big as the WMAP cold spot is exceedingly unlikely, according to standard cosmology," says Mersini-Houghton.

There are other explanations for the WMAP cold spot. For example, some researchers speculate that it is due to a giant knot in space called a topological defect, predicted in certain theoretical models (New Scientist, 13 July, p 12). However, Mersini-Houghton's explanation could have greater significance.
Entangled universes

She and her colleagues looked for an explanation outside of standard cosmology. They turned to string theory, the leading contender for a "theory of everything" that unites the laws of physics to explain how all matter and energy behaves. The theory holds that the ultimate building blocks of matter, such as quarks and leptons, are tiny strings of mass-energy vibrating in a 10-dimensional space-time.

String theory's selling point had always been that it could make unique predictions about the properties of our universe. This made it more aesthetically pleasing than anthropic arguments, which say that certain aspects of the universe - like the constants that characterise the laws of physics - are the way they are because otherwise we wouldn't be here to wonder about them.

Yet string theory does not just describe one universe. It describes 10500 universes, each one a quantum vacuum with different physical properties. So why was ours the universe that grew large? "String theorists, who so much hoped to avoid the anthropic principle, have now been forced to invoke it to explain why our vacuum was selected out from the 10500 other string vacuums," says Mersini-Houghton.

Anthropic arguments leave many physicists queasy. They would prefer an explanation for the universe's properties that has nothing to do with our existence. Rather than abandon string theory completely, however, Mersini-Houghton was convinced there must be a way to thin down the forest of string vacuums without using the anthropic principle. She and her collaborator Richard Holman of Carnegie Mellon University in Pittsburgh, Pennsylvania, had a hunch that matter and gravity might have some kind of dynamic effect that whittles down the number of vacuums to a small bunch that eventually grows into our universe and its neighbours.

According to string theory, each possible universe has different conditions. If a patch of vacuum is to lead to a universe like ours, the important thing is that it must grow large. This means something must oppose gravity, which tends to suck together the mass-energy of the vacuum and shrink it.

That something can only be the vacuum itself. If the vacuum has an enormous negative pressure, Einstein's theory of gravity says it will generate repulsive gravity that blows rather than sucks. "A patch of vacuum's repulsive gravity therefore overwhelms the attractive gravity of its matter," says Mersini-Houghton. "For the patch of vacuum that led to our universe, this happened during the first split second of its existence in a period called inflation."

According to Mersini-Houghton and Holman, the dynamic effect of matter and gravity would have weeded out the majority of string vacuums, leaving only our patch and close neighbours in the string landscape. "It's a much more scientific and legitimate way of picking out a universe like ours than the anthropic principle," she says. "But it has extraordinary consequences."

Mersini-Houghton and Holman's calculations show that the patch of vacuum that led to our universe must have interacted with neighbouring patches very early on. Because these interactions are between tiny patches of quantum vacuum, they would leave the universes in an entangled state and give them a ghostly connection that allows them to sense and affect each other from afar. "Such an entangled state remains for all time," says Mersini-Houghton. "So although inflation quickly pushed our region beyond the reach of neighbouring regions, it should still retain the imprint of its quantum entanglement with its neighbours."

The question is: where should we look for the imprint and what form might that imprint take? Because of the expansion of the universe, no light or signals can reach us from beyond the cosmic horizon, about 42 billion light years away. On a far smaller scale, the messy process of galaxy formation has effectively erased any trace of the early interaction between our universe and neighbouring ones. However, on scales comparable to the cosmic horizon itself, there ought to remain an imprint from the time closest to the beginning of inflation when there was an interaction. "In today's universe, it should manifest itself at a red shift of less than 1, corresponding to a time when the universe was about half its present age, says Mersini-Houghton."
Smoking gun

Mersini-Houghton and Holton say their dynamical theory can describe the form of the imprint too. The vacuums of neighbouring patches effectively push on our universe, they say. According to relativity, such squeezing produces repulsive gravity. Where we can see the squeezing act - on scales comparable with the size of the universe - the repulsive gravity should dramatically thin out matter and make it harder for galaxies to form. "We predict the existence of a giant void about 500 million light years across," says Mersini-Houghton. By cosmology's standards this forecast ties in pretty well with astronomers' observations of a void 900 million light years across at a red shift of 1. "We are amazed that the void is there just as we predicted," she says.

Working with Tomo Takahashi of Saga University in Japan, Mersini-Houghton and Holman go further. They predict that there should be not one such giant void but two: one in the northern hemisphere corresponding to the WMAP cold spot and one in the southern hemisphere. "We are hoping that a southern void will turn up in the data soon," she says.

So far the work has had a mixed reception. "It is one of the most interesting ways to relate observations in our universe to the vastly larger string landscape," says physicist Leonard Parker of the University of Wisconsin, Milwaukee. Others are more cautious. "It's interesting," says David Spergel of Princeton University. "However, it is very speculative."

Mersini-Houghton and her team make a further prediction that could soon be tested - what they call the "smoking gun" of their idea. In standard cosmology, the temperature variations of the big bang radiation are the direct result of the distribution of matter in the universe. This means the pattern of galaxies should exactly match the temperature features in the big bang radiation.

That won't be the case, says Mersini-Houghton. Her team's work shows that the entanglement between our universe and neighbouring universes changes the density of matter on the largest scales. If they are right, the interaction will leave a subtle mark on observations. "We predict that correlation between matter and temperature will be found to be much less than 100 per cent."

The test could come as soon as next year, when the European Space Agency launches its Planck microwave background probe. Planck should be able to both confirm the existence of the cold spot and improve the precision of the WMAP sky map.

Planck isn't the only test. Mersini-Houghton also makes a prediction about what will be seen - or rather not seen - at the Large Hadron Collider (LHC) near Geneva, Switzerland, when it switches on next year. Many particle physicists believe that the LHC will uncover the first experimental evidence for supersymmetry, a popular theory that posits that every particle has a heavier superpartner. None of the particle accelerators built so far has had enough energy to create supersymmetric particles, but physicists believe that the collision energy at the LHC will produce fireballs with sufficient energy to recreate conditions in the early universe.

They hope to test what happened when the universe cooled below a certain temperature and underwent a phase transition, which broke supersymmetry. According to string models, the energy released during the phase transition drove inflation, and went on to create supersymmetric particles. Since the energy had to be sufficient to ensure the growth of our piece of vacuum, Mersini-Houghton and her colleagues can make an estimate of the energy scale of supersymmetry breaking. "We find it is about 100,000 times greater than generally believed," she says. "Therefore we predict that the LHC will not detect supersymmetry."
String theory's saviour

It is a controversial result and many physicists disagree. "The string landscape is quite dense and it is most likely that vastly different physical parameters may give rise to quite similar universes," says Orfeu Bertolami of the Instituto Superior Técnico in Lisbon, Portugal. "Nevertheless, I find their work very interesting."

Despite the disagreement, the latest work is emblematic of a recent U-turn in theoretical physics. When the first WMAP results were made public in 2002, cosmologists claimed that the findings confirmed the standard model of the universe. Nobody expected anomalies to emerge and, if they did, nobody expected they would threaten to turn the standard picture of cosmology on its head.

Worse, some physicists have started to turn their backs on string theory in recent years, fearing that it is a dud, incapable of making any testable predictions. Some have even gone as far as declaring string theory dead. "I think our evidence points to string theory being on the right track," says Mersini-Houghton. Now, with the discovery of the hole in the universe, it seems it could be a case of string theory is dead, long live string theory.