How to Destroy the Universe (19 page)

For this reason, many scientists have interpreted Kerr black holes as wormholes—tunnels through space and time (see
How to travel through time
). Virtually all objects in space have some degree of rotation—think of planets, the Solar System, and the galaxy—meaning that Kerr black holes may well be the norm rather than the exception. If that's the case, then falling into a black hole might not always be the death sentence it's often made out to be. Far from it—when it comes to bridging the gap between our Universe and others, it could be the safest way to travel.

• Eddington's eclipse

• Gravitational lensing

• The ultimate telescope

• Microlensing

• Multiply-connected universes

Today, telescopes on Earth, and in orbit around it, are so powerful they could resolve the gap between two car headlamps on the Moon. But sometimes even these mighty instruments need a helping hand. A phenomenon called gravitational lensing—where the light from distant galaxies is focused by gravity—is boosting the magnification of terrestrial telescopes to reveal objects tens of billions of light years away, at the very edge of space.

In 1919, the British astronomer Sir Arthur Eddington led an expedition to the African island of Principe to view a total eclipse of the Sun. But this was no mere sightseeing trip. Eddington was there to test one of the
most radical theories in physics ever put forward. Called the general theory of relativity, and authored by one Albert Einstein, the theory supposed that the gravitational interaction between objects is caused by the deformations their masses cause to the structure of space and time. If this was correct then anything passing through space should experience the distortion—including beams of light. On the other hand, the existing theory of gravity, put forward by Isaac Newton, made no such claim over light.

Eddington proposed to test Einstein's theory by measuring the degree to which a light ray gets bent by the gravity of the Sun. The biggest light-bending effect would occur where the Sun's gravitational field is strongest: when a light ray just skims across its surface. But there was a problem. The Sun is extremely bright and so any such rays, say from distant stars, will be swamped by the solar glare. Except, that is, during a total eclipse. When this happens the Moon passes in front of the Sun. By sheer coincidence, the angular size of the Sun and Moon are identical. Even though the Moon is physically smaller than the Sun, it's much closer to us—by just the right amount so that its disc at totality exactly obscures the disc of the Sun. Eddington planned to take advantage of this darkness during the total eclipse to measure the effect of the Sun's gravity on the apparent positions of stars nearby in the sky. His observations were almost thwarted by bad weather, but
Eddington was successful in taking a number of pictures of the eclipse, and these confirmed the light-bending effect—showing the degree of bending to be in good agreement with the predictions of general relativity.

If light bends around the Sun then it should also be bent by other massive objects in space, such as galaxies. In 1924, Russian physicist Orest Chwolson published an article outlining how the same effect should take place on vast cosmic scales so that the light from very distant galaxies gets distorted by the gravity of bodies along the line of sight to Earth. Chwolson pointed out that if the intervening body is exactly on the line joining the distant galaxy to the observer, the distant galaxy will appear not as a point but as a ring.

To see why, imagine for a second that space is like a two-dimensional sheet of paper. Now draw a straight line on the paper and mark along it three points: planet Earth, the distant galaxy and, exactly between the two, the intervening mass doing the lensing. Rather than traveling directly along the straight line, the gravitational distortion makes the light from the galaxy travel to Earth along two curved arcs, one above the line and one below it. A two-dimensional astronomer on Earth would see two images of the galaxy either side of its true position on the sky, one arriving along each arc.

To see what happens in three dimensions, simply rotate the two-dimensional diagram around the axis formed by the original straight line. The arcs along which light travels now sweep out a shape resembling a rugby ball, and the two images that are formed in two dimensions now trace out a ring. The light from the galaxy is curved around the intervening mass in much the same way as light passing through a lens. And for this reason the process has come to be known as gravitational lensing. If the mass doing the lensing is slightly off axis, then the perfect ring breaks up into a number of images of the distant galaxy, stretched out into arc-like segments. However, Chwolson's research was not a particularly in-depth analysis. Albert Einstein picked up the baton in 1936, writing one of the definitive papers on gravitational lensing, and building an extensive mathematical framework by which astronomers could calculate the degree of bending produced as a function of distance and the masses involved. The bright ring produced by a perfectly lensed galaxy is now usually known as an “Einstein ring.” The first real one was found in 1998 by a team using NASA's Hubble Space Telescope.

The first such gravitational lens was discovered in 1979, when astronomers spotted a double image of a faraway type of galaxy known as a quasar. Quasars are fiercely bright galaxies seen at the edge of the visible
Universe. Hundreds of thousands of them are now known, though none is closer than about 3 billion light years to Earth. Because of the look-back time—the time taken for their light to reach us—this means we're seeing quasars as they were at least 3 billion years ago, suggesting that these objects were perhaps a phase in the evolution of young galaxies that has now finished. Quasars were discovered in 1963 by Dutch–US astronomer Maarten Schmidt. The name is a contraction of “quasi-stellar object,” which they were given because it wasn't immediately recognized that they were galaxies. Their great distance from Earth had made them appear as points of light in the sky, so that they looked more like stars (hence “stellar objects”).

The double quasar that marked the discovery of gravitational lensing was 8.7 billion light years away. If that seems a lot, the farthest quasar ever seen (as of early 2010) is an astonishing 28 billion light years away. Gravitational lenses don't just distort the light from faraway galaxies and quasars but they magnify it too. Light rays which ordinarily would have meandered away into empty space are captured by the intervening cluster's gravity and hooked around onto an intercept course with Earth. More light from the quasar reaches Earth than would do were it not lensed. This enables us to see quasars that are further away than we would normally be able to see.

But this advantage comes at a price. For any particular quasar, the chances of there being a massive galaxy or cluster of galaxies directly on or close enough to our line of sight for lensing to happen are slim, and so the number of lensed quasars observed is very small compared to the total number of quasars that are out there. For many other distant quasars, the gravity of other galaxies and galaxy clusters must pull light away from a path that would ordinarily intercept Earth, rendering these objects too faint for us to detect.

It's not just objects on the other side of the Universe that astronomers can study using gravitational lensing. The magnification effect that it causes makes it an excellent way for spotting the passage of objects within our galaxy that would otherwise be invisible to telescopic eyes. The object itself is not magnified, but rather it causes magnification of the light from background stars as it passes by.

One class of faint object this technique is used to search for are so-called brown dwarf stars. These are failed stars, which were never massive enough to create the temperature and pressure at their cores needed to spark up nuclear fusion reactions (see
How to create life
). Physically, these objects are thought to resemble the planet Jupiter—vast balls of gas, but not emitting any
light or other radiation. Because they are dark, brown dwarfs are extremely hard for astronomers to spot in the blackness of space. Until, that is, they happen to wander in front of a background star. When that happens the mass of the brown dwarf gravitationally lenses the light from the star, causing it to momentarily grow brighter. The mass of a brown dwarf is typically less than 7 percent the mass of the Sun or, equivalently, about 75 Jupiter masses. This means that the lensing effect is small, which is why it is known as microlensing.

Studying how the light from a lensing event grows and fades can reveal information about the object doing the lensing, such as its size, speed and distance. In 1998, researchers at the University of Sussex even suggested that microlensing might be one way to look for wormholes. These are tunnels through space and time, whose existence is predicted by general relativity (see
How to travel through time
). Wormholes require a special kind of material to hold them open, which has negative mass and so generates a kind of repulsive gravity. This deflects the light rays around the wormhole in a markedly different way from how the rays are normally bent around, say, a brown dwarf, fanning them out rather than focusing them inwards, to make a double-peaked lensing event. The first peak reaches maximum brightness slowly and then darkens rapidly, to be followed a few years later by a second peak that
brightens rapidly and then fades out gradually. Astronomers could in principle look for this behavior in the light from microlensing events to discover wormholes—objects which no one has yet seen in the real world.

According to one slightly bizarre theory of the Universe, if you use a gravitational lens (or any other kind of powerful telescope) to peer into the outer reaches of space, you could actually end up peering in on somewhere a little closer to home.

In a so-called multiply-connected universe, space is wrapped around on itself so that light rays traveling far enough in one direction ultimately arrive back where they started. The possibility arises because Einstein's theory of general relativity says nothing about a property of space called topology. Broadly speaking, this determines the overall shape of space and how different regions are connected to one another. For example, a flat sheet of paper has different topology from the surface of a sphere—because on a sphere you can travel all the way around without ever arriving at the edge. Glue opposite sides of the sheet of paper together and you get a different kind of topology again, resembling a doughnut. Now you can travel on a loop either around the outside of the doughnut or through the hole in the middle and arrive back where
you started. However, it is possible to set a course that spirals around the ring and never quite brings you back to your starting point.

There are far more complicated kinds of topology than spheres and doughnuts. In 2003, a French-led team suggested that our Universe could have a weird topology based on a 12-sided dodecahedron, arranged so that if you exit on one face you re-enter through the face opposite. They claimed to have found tentative evidence for their theory in the pattern left behind in the cosmic microwave background radiation—the microwave echo of the Big Bang fireball in which our Universe began.

The claim is controversial. However, space probes expected to launch over the coming decades to measure the microwave background in unprecedented detail could provide the conclusive evidence one way or the other. After all of the astronomers' efforts to gaze ever further out into the blackness of space, they could ultimately end up zooming in on the backs of their own heads.

• The Big Bang

• The microwave background

• Particle cosmology

• Particle accelerators

• Hunt for the Higgs

• The end of the world?

Some fear it will destroy the world. Scientists, on the other hand, say it will utterly revolutionize our view of the cosmos. Either way, the Large Hadron Collider particle accelerator, on the southern border of France, is the most complex machine ever built by human beings. It will accelerate subatomic particles to 99.99 percent of light speed and slam them together, generating temperatures over 100,000 times hotter than the Sun's core in an effort to reconstruct the fiery conditions of the Big Bang in which our Universe was born.

Somewhere around 13.7 billion years ago, something incredible happened. Out of the void of total and
complete nothingness—no matter, no radiation, not even any space or time—our Universe popped into existence in a quantum event known as the Big Bang. The cosmos was born into a state known as a gravitational singularity—a point of zero size and infinite density, temperature and pressure. By rights this primeval maelstrom should have collapsed back and disappeared again as suddenly as it appeared. But the matter that had just been brought into existence had other ideas. About one hundred-million-billion-billion-billionth of a second after its creation, the stuff of the embryonic universe underwent a phase transition—a wholesale shift in its properties, rather like the conversion of steam into water as it cools. Unlike water, though, this change was to have dramatic consequences. It filled space with a kind of anti-gravitating material—rather like the “dark energy” that's thought to pervade the Universe today (see
How to destroy the Universe
). This caused the Universe to expand stupendously fast, increasing in size by a factor of 10
²⁶
—a 1 followed by 26 zeroes. To put that in perspective, if at the start of this phase of cosmic “inflation,” as cosmologists call it, the Universe was about the size of a tennis ball, then by the end of inflation that tennis ball would have grown to be about a billion light years across: about the distance to the furthest galaxies observable in the Universe today.