杏吧原创

AT THE beginning of the Universe, there wasn鈥檛 much information around. The hard drive of your personal computer could probably store everything there was to say about the entire early cosmos. But that scarcity of information doesn鈥檛 mean we can never know what the beginning looked like 鈥� indeed, it means precisely the opposite. Because there was so little information, it is possible that we may soon have everything we need to observe the first moments of existence.

Because it takes time for light to travel through the Universe, the farther we peer into space, the farther back in time we are looking. Our views of the distant past therefore come from images of the distant Universe. The most distant views we have so far reach back to almost the start of the big bang. So could we see the beginning of time itself? If so, what would it look like? And what might we learn about how time and space emerged from, well, whatever it was they emerged from?

The fundamental nature of time is tied up in the search for 鈥渜uantum gravity鈥�, a unified description of nature that will finally reconcile our two best theories so far, relativity and quantum theory. Relativity describes how gravity works, but applies only on large scales. Quantum theory describes phenomena on mainly subatomic scales, and explains how matter and energy behave.

Since matter and energy always come in discrete packets or 鈥渜uanta鈥�, such as electrons or photons of light, physicists suspect that gravitational energy also has a fundamentally quantum character, and hence so do space and time. And because theorists believe nature is mathematically consistent, they are looking for a theory of quantum gravity to describe how time, space, matter and energy interact.

Modern approaches to quantum gravity, such as string theory, suggest that at small enough scales, space-time might not be divisible any further 鈥� that it might make no sense to speak of infinitesimally small times or distances. Space-time may not be a continuum but could instead be built of fundamentally discrete elements, or quanta. Thus instead of evolving in a continuous manner, somehow everything jumps from one 鈥渟tate鈥� to the next.

And this is what we may soon be able to see by looking at the oldest light we can detect: the cosmic microwave background. Discovered in 1965, it is a faint glow of cooled radiation that permeates all of space, a relic from the hot early stages of the big bang. This afterglow arrives so nearly uniformly from all directions that only in the past 10 years have scientists succeeded in mapping its slight temperature variations across the sky. These maps reveal an all-important mottled pattern of very slightly warmer and cooler blobs.

We now have several such maps to compare, thanks to the Cosmic Background Explorer (COBE) satellite, experiments carried on various high-altitude balloons, and ground-based radio telescopes. But we are about to get our best measurements yet from NASA鈥檚 Microwave Anisotropy Probe. MAP鈥檚 results are being released this month and will provide extremely high-resolution images of the whole sky 鈥� and our best view of the early Universe.

The slight fluctuations in temperature that show up on these microwave maps are believed to have begun as single quantum variations during the Universe鈥檚 formative inflationary period. They are a visible relic of quantum noise, a kind of tiny fluctuation in the energy fields of the early Universe. As such, the microwave maps provide faithful images of the smallest things in nature. And yet, bizarrely, they are simultaneously the largest things we can, or ever will be able to see. That鈥檚 because the expanding Universe has stretched them from subatomic to cosmological size. Like a giant movie projector, the expansion has thrown hugely magnified images of these primordial quanta onto the sky.

In the same way that observing discrete spectral lines in light gave the first hint of the deep quantum-mechanical structure of atoms, these quanta may imprint some kind of discreteness in the high-resolution images from MAP. If they do, it would give us our first view of space-time鈥檚 quantum structure as it emerged in the early Universe.

Using this data to observe the 鈥渂eginning of time鈥� is still a highly speculative idea. The microwave maps have already been extremely useful in firming up our understanding of the Universe鈥檚 structure (see 鈥淨uantum cosmos鈥�), but our theories of time are still in a primitive state. We don鈥檛 know whether time had a beginning, or is infinite and predates our expanding Universe, or emerged from some more fundamental entity. However, remarkable new ideas hint that observable features might indeed survive from when time first emerged as time in our Universe.

It is all to do with information, or rather, a lack of it. All systems have a certain information content, which roughly speaking is equal to the length of the binary number needed to enumerate all the possible states of the system. Meanwhile, work on the physics of black holes has shown that information has surprisingly deep connections with space, time and gravity (see 鈥淥ut of the darkness鈥�). Pursuing these ideas has led Leonard Susskind of Stanford University and Gerard鈥檛 Hooft of the University of Utrecht to propose a 鈥渉olographic principle鈥� for information: they suggest that the information needed to describe everything in any 3D volume of space is proportional to the area of its 2D outer surface, just as a hologram encodes a 3D image on a 2D surface (New 杏吧原创, 27 April, p 22).

The holographic principle puts a surprisingly low upper bound on the amount of information that can be stuffed into any region of quantised space-time. Suppose you have a two-dimensional surface such as a sphere. Imagine that it is covered with pixels, each with an area 0.724 x 10^鈭�65 cm² (the 鈥淧lanck area鈥�, a number that comes from combining Newton鈥檚 gravitational constant with the quantum-mechanical 鈥淧lanck constant鈥�). Then the number of possible ways that events can happen in the volume inside the sphere is the same as the number of ways you can fill all of the pixels on the surface with either a 0 or a 1.

And so, according to Susskind and 鈥榯 Hooft鈥檚 鈥渉olographic bound鈥�, the whole panoply of possible physics in any three-dimensional region 鈥� such as the observable Universe 鈥� can be encoded by a limited set of numbers on its two-dimensional boundary. As 鈥榯 Hooft puts it, 鈥淭his is what we found out about nature鈥檚 book keeping system: the data can be written onto a surface, and the pen with which the data are written has a finite size.鈥�

The holographic principle is a radical conjecture that implies a fundamental limit to nature鈥檚 capacity to do things, and a limit to the potential for complexity or indeed behaviour of any kind. The principle means that the state of the whole observable Universe today can be specified with about 10¹²⁰ binary digits 鈥� the number of Planck areas on the boundary surface of our observable volume. This is a truly huge number, far more than the total number of particles and photons in the Universe, which is about 10⁸⁷. For all practical purposes, space and time today behave like a continuum because they can contain so much information. That makes it hard to find ways to observe the effect of this information bound.

However, the observable Universe was far smaller when it gave rise to the quanta that we now see as blobs on the microwave maps. It was bounded by a surface as small as perhaps only 10¹⁰ Planck areas. And so the total amount of information in the Universe was much smaller than today 鈥� small enough that the quantum nature of space-time might possibly leave an observable imprint.

This idea derives from empirical measurements of the quantum fluctuations. The size of the temperature fluctuations in the microwave maps is only about a thousandth of one per cent of their mean temperature. It鈥檚 fair to suppose that those perturbations carry a similarly tiny proportion of the total information. As the Universe expanded, its information content was increasing all the time. We know that during the time it took to form each quantum fluctuation the total information content of the Universe changed by about 10¹⁰ bits, or about a gigabyte. So the observed quanta will contain about a thousandth of one per cent times that: about 10⁵ bits each. This limited information content was frozen into the fluctuations and so the regions of the quantum fluctuations still contain at most only about 10⁵ bits. That is certainly less than the number of bits in MAP鈥檚 images of these structures, so we may see some effects from this lack of information.

What would this look like? We are all familiar with degradation of sounds and images when the source doesn鈥檛 contain enough information, such as the pixelation of security camera footage, or overblown, low-resolution digital photographs. The beginning of time 鈥� or, more accurately, the beginning of space-time 鈥� might look something like this: the smooth noise signal might start 鈥渂reaking up鈥� into some pattern imposed by the fundamental elements out of which space-time is formed.

Thus, if we knew what to look for, a perfect map of the microwave background would be noticeably ordered in some way, the equivalent of graininess. However, knowing what to look for is far from easy. With good compression software 鈥� the right kind of encoding 鈥� a rather high-quality picture can be described with a relatively small amount of data, which makes it hard to see any degradation even though there is relatively little information. Since we don鈥檛 have a theory of quantum gravity yet, we don鈥檛 know the 鈥渆ncoding鈥� for a holographic universe. All we can do right now is estimate the number of possible states, equivalent to the file size, but even a small file size doesn鈥檛 guarantee us a grainy image.

There鈥檚 another potential obstacle to our view of the beginning of time. Any real map, including MAP鈥檚, suffers from noise generated by the experiment itself. The COBE map was the first one to have low enough instrument noise to even detect the primordial noise. Although the new maps to be released this month will be much better, the holographic graininess would have to be rather coarse for us to see primordial quantisation amid the instrument noise. To be almost guaranteed to see it, we would need maps with at least one-thousandth the noise MAP will generate.

But, although we have no guarantees, MAP may just have what we鈥檙e looking for. If we are lucky, we might find some kind of holographic graininess in MAP鈥檚 rich dataset. With such an advance in sensitivity and size as MAP affords, we should at least be alert to the possibility that something qualitatively new and unexpected might appear 鈥� perhaps even the stuff from which time and space emerge. The blobby patches in the MAP images to be released this month will be our best view of what are all at once the smallest, the biggest, and the most distant things. And for the time being at least, they are as close as we get to seeing the beginning of time.