快猫短视频

EVER since 1905, when Albert Einstein revealed his special theory of
relativity to the world, the speed of light has had a special status in the
minds of physicists. In a vacuum, light travels at 299 792 458 metres per
second, regardless of the speed of its source. There is no faster way of
transmitting information. It is the cosmic speed limit. Our trust in its
constancy is reflected by the pivotal role it plays in our standards of
measurement. We can measure the speed of light with such accuracy that the
standard unit of length is no longer a sacred metre bar kept in Paris but the
distance travelled by light in a vacuum during one 299 792 458th of a
second.

It took cosmologists half a century to the full cosmological importance of a
finite speed of light. It divides the Universe into two parts: visible and
invisible. At any time there is a spherical 鈥渉orizon鈥� around us, defined by the
distance light has been able to travel since the Universe began. As time passes,
this horizon expands. Today, it is about fifteen billion light years away.

This horizon creates a host of problems for cosmologists. Because no signals
can travel faster than light, it is only within the horizon that light has had
time to establish some degree of uniformity from place to place in terms of
density and temperature. However, the Universe seems more coordinated than it
has any right to be. There are other ways, too, in which the Universe seems to
have adopted special characteristics for no apparent reason. Over the years,
cosmologists have proposed many different explanations for these
characteristics鈥攁ll with their attendant difficulties. In the past year,
though, a new explanation has come to light. All you have to do is break one
sacred rule鈥攖he rule that says the speed of light is invariable鈥攁nd
everything else may well fall into place.

The first of the problems cosmologists need to explain is a consequence of
the way the cosmological horizon stretches as the Universe expands. Think about
a patch of space which today reaches right to the horizon. If you run the
expansion of the Universe backwards, so that the distances between objects are
squeezed smaller, you find that at some early time T after the big bang that
same patch of space would lie beyond the horizon that existed then. In other
words, by time T there would not have been enough time for light to have
travelled from one edge of the sphere bounded by our present horizon to the
opposite side.

Because of this, there would have been no time to smooth out the temperature
and density irregularities between these two patches of space at opposite
extremes of our present horizon. They should have remained uncoordinated and
irregular. But this is not what we see. On the largest cosmic scales the
temperature and density in the Universe differ by no more than a few parts in
one hundred thousand. Why? This is the horizon problem.

Another, closely related cosmological problem arises because the distribution
of mass and energy in our Universe appears to be very close to the critical
divide that separates universes destined to expand for ever from those that will
eventually collapse back to a 鈥渂ig crunch鈥�. This is problematic because in a
universe that contains only the forms of matter and radiation that we know
about, any deviation from the critical divide grows larger and larger as time
passes. Our Universe has apparently been expanding for nearly 15 billion years,
during which time its size has increased by a factor of at least 10³². To have
remained so close to the critical divide today the Universe must have been
incredibly close to this distribution of mass and energy when it started
expanding鈥攁n initial state for which there is no known justification. This
is the flatness problem, so called because the critically expanding state
requires the geometry of space to be flat rather than curved.

The third major problem with the expansion of the Universe is that Einstein鈥檚
theory of gravitation鈥攇eneral relativity鈥攁llows the force of gravity
to have two components. The better known one is just a refinement of Newton鈥檚
famous inverse-square force law. The other component behaves quite differently.
If it exists, it increases in direct proportion to the distance between objects.
Lambda was the Greek symbol used by Einstein to denote the strength of this
force in his theory. Unfortunately, his theory of gravitation does not tell us
how strong this long-range force should be or even whether it should push masses
apart rather than pull them together. All we can do is place stronger limits on
how big it is allowed to be, the longer we fail to see its effects.

Particle physicists have for many years argued that this extra component of
the gravitational force should appear naturally as a residue of quantum effects
in the early Universe and its direction should be opposite to that of Newton鈥檚
law of gravity: it should make all masses repel one another. Unfortunately, they
also tell us that it should be about 10¹²⁰ times larger than astronomical
observations permit it to be. This is called the lambda problem.

Expanding fast

Since 1981, the most popular solution to the flatness and horizon problems
has been a phenomenon called inflation that is said to have occurred very soon
after the big bang, accelerating the Universe鈥檚 expansion dramatically for a
brief interval of time. This allows the region of the Universe seen within our
horizon today to have expanded from a much smaller region than if inflation had
not occurred. Thus it could have been small enough for light signals to smooth
it from place to place. Moreover, by the end of this bout of acceleration the
expansion would be driven very close to the critical divide for flatness. This
is because making a curved surface very large ensures that any local curvature
becomes less noticeable, just as we have no sense of the Earth鈥檚 curved surface
when we move a short distance.

Why should the Universe have suddenly inflated like this? One possibility is
that strange, unfamiliar forms of matter existed in the very high temperatures
of the early Universe. These could reverse the usual attractive force of gravity
into repulsion and cause the Universe to inflate briefly, before decaying into
ordinary radiation and particles, while the Universe adopted its familiar state
of decelerating expansion.

Compelling as inflation appears, it cannot solve the lambda problem. It has
also had to confront some new observations of the rates at which distant
supernovae are receding from us. These imply that the lambda force is
influencing the expansion of the Universe today
(鈥淭he fifth element鈥�, 快猫短视频, 3 April).
Even though the density of matter might be just 10 per
cent of the critical value, the influence of the lambda force means the geometry
of space might still be very close to flatness. If these observations are
corroborated, they make the flatness and lambda problems worse: why is the
Universe quite close to the critical rate of expansion (1part in 5, say, rather
than 1 part in 100 000) and why is lambda finite and having a similar influence
on the expansion of the Universe as the matter in the Universe today? Since
these two influences change at different rates as the Universe ages it seems a
very weird coincidence that they just happen to be similar in strength today
when we are here to observe them. These are called the quasi-flatness and
quasi-lambda problems, respectively.

Last year, with a view to providing some alternative to inflation, Andreas
Albrecht of the University of California at Davis, and Jo茫o Magueijo of
Imperial College, London, investigated an idea first suggested by John Moffat, a
physicist at the University of Toronto. Moffat had proposed that the speed of
light might not be such a sacrosanct quantity after all. What are the
cosmological consequences if the speed of light changed in the early life of the
Universe? This could happen either suddenly, as Albrecht, Magueijo and Moffat
first proposed, or steadily at a rate proportional to the Universe鈥檚 expansion
rate, as I suggested in a subsequent paper.

The idea is simple to state but not so easy to formulate in a rigorous
theory, because the constancy of the speed of light is woven into the warp and
weft of physics in so many ways. However, when this is done in the simplest
possible way, so that the standard theory of cosmology with constant light speed
is recovered if the variation in light speed is turned off, some remarkable
consequences follow.

If light initially moved much faster than it does today and then decelerated
sufficiently rapidly early in the history of the Universe, then all three
cosmological problems鈥攖he horizon, flatness and lambda problems鈥攃an
be solved at once. Moreover, Magueijo and I then found that there are also a
range of light-slowing rates which allow the quasi-flatness and quasi-lambda
problems to be solved too.

So how can a faster speed of light in the far distant past help to solve the
horizon problem? Recall that the problem arises because regions of the Universe
now bounded by our horizon appear to have similar, coordinated temperatures and
densities even though light had not had time to travel between them at the
moment when these attributes were fixed. However, if the speed of light were
higher early on, then light could have travelled a greater distance in the same
time. If it were sufficiently greater than it is today it could have allowed
light signals to traverse a region larger than would expand to fill our horizon
today (see Figure).

As regards the flatness problem, we need to explain why the energy density in
the Universe has remained at the critical divide that yields a flat, Euclidean
space, even though its expansion should have taken it farther and farther from
this divide. And as for the lambda problem, we need to explain why the lambda
force is so small鈥攊nstead of the huge value that particle physicists
calculate.

The key point here is that the magnitude of the expansion force that drives
the Universe away from the critical divide, and the magnitude of the lambda
force, are both partially determined by the speed of light. The magnitude of
each is proportional to the square of the speed of light, so a sufficiently
rapid fall in its value compared with the rate of expansion of the Universe will
render both these forces negligible in the long run. The lambda force is harder
to beat than the drive away from flatness. Consequently, a slightly faster rate
of fall in light speed is needed to solve the flatness, horizon, and lambda
problems than is required just to solve the flatness and horizon problems.

Remarkably, a more modest slowing of light allows the quasi-flatness problem
to be solved: it leads to a Universe in which the forces that drive the Universe
away from a critical state ultimately keep pace with one another, neither
overwhelming the other. In the same way, a suitable rate of change of light
speed can result in an approach to a critical rate of expansion in which the
lambda force keeps pace with the gravitational influence of matter.

One advantage that the varying light speed hypothesis has over inflation is
that it does not require unknown gravitationally repulsive forms of matter. It
works with forms of matter and radiation that are known to be present in the
Universe today. Another advantage is that it offers a possible explanation for
the lambda problem鈥攕omething inflation has yet to solve.

In pursuit

The simplicity of this new model and the striking nature of its predictions
suggest that we should investigate it more seriously. We should find the most
comprehensive formulation of gravity theories that includes a varying speed of
light and that recovers existing theories when that variation is turned off;
search for further testable predictions of these theories; and pursue
observational evidence for varying constants that depend on light speed.

The standard picture of inflation makes fairly specific predictions about the
patterns of fluctuations that should be found in temperature maps of the
radiation left over from the early stages of the Universe. Future satellite
missions, including NASA鈥檚 MAP satellite鈥攄ue for launch next
year鈥攁nd the European Space Agency鈥檚 Planck Surveyor, due for launch
several years later, will seek out those fluctuations to see if they match the
predictions. Now we need to work out if a past variation of the speed of light
makes equally specific predictions.

In recent years, dozens of theoretical physicists have been studying the
properties of new superstring theories that attempt to unite the fundamental
forces of nature within a quantum gravitational framework. They have revealed
that traditional constants of nature, such as Newton鈥檚 gravitational constant or
the fine structure constant鈥攆ormed by dividing the square of the electric
charge on a single electron (e²) by the product of the speed of light (c) and
Max Planck鈥檚 quantum constant (h/2&pgr;)鈥攄o not need to be quite as constant
as we thought. If extra dimensions of space exist, as these theories seem to
require, then any change in those extra dimensions will produce variations in
the constants that underpin our three-dimensional space.

Another exciting possibility for astronomers to check is whether variations
in constants that involve the speed of light could still be observable today and
not just confined to the first split second of cosmic history. Recently,
Magueijo and I have found that a tiny residual effect may remain in the
Universe鈥攕imilar in form to that revealed by the supernova
observations鈥攍eft over from a significant variation of the speed of light
in its very early stages. Since laboratory experiments are not sensitive enough
to detect such small variations, we must look to astronomy for a probe.

Two years ago, John Webb, Michael Drinkwater and Victor Flambaum鈥攁ll
then at the University of New South Wales, Sydney鈥攁nd I looked at the
spectra of carbon monoxide molecules and hydrogen atoms in gas clouds. Because
of the finite speed of light, looking at distant cosmological objects is
equivalent to looking back to earlier times in the Universe鈥檚 history. We
checked whether the ratios of energy levels of these atoms and molecules were
intrinsically different at the time of the gas cloud compared with their values
on Earth today. These ratios depend on the square of the fine structure constant
and we found it to be constant to better than 5 parts in a million鈥攁 limit
a hundred times better than that found by direct laboratory experiments.

Highly sensitive

More recently, joined by Chris Churchill of Pennsylvania State University in
College Park, we devised a very sensitive technique for comparing relativistic
atomic transition frequencies between iron and magnesium in the spectra from 30
quasars. For the closest and most distant quasars, we confirmed the limits set
by the gas clouds, but those in a narrow range of distances in between display a
shift that is consistent with a variation in the fine structure constant. This
shift could also be caused by a problem that astronomers call 鈥渓ine blending鈥�.
Further data have been gathered and should unambiguously reveal the source of
the observed shift.

New telescopes open up the exciting possibility of measuring physical
constants far more stringently than is possible in laboratory experiments. The
stimulus provided by superstring-inspired theories of high-energy physics,
together with the theory that a change in the speed of light in the early
Universe may have propelled it into the peculiar state of near smoothness and
flatness that we see today, should provoke us to take a wide-ranging look at the
constancy of nature鈥檚 鈥渃onstants鈥�. Tiny variations in their values may provide
us with the window we are searching for into the next level of physical
reality.