The mysterious dark matter thought to make up most of the
Universe's mass could be made of particles 10 trillion times
heavier than a proton, new research suggests. If so, detectors
might one day trap dark matter in the form of an extremely heavy
type of hydrogen. More than 90 per cent of the matter in the
Universe is invisible, and can be detected only by its gravitational
effects on stars and galaxies. Now theoretical work done by Edward
Kolb of Fermilab, near Chicago, suggests that the dark matter
sprang into being as superheavy particles moments after the Universe
formed. The particles would have been created at the end of the
inflationary epoch, the period of tremendous growth during the
first split second of the Universe's existence. The inflationary
epoch was characterised by huge negative pressure. Einstein's
general theory of relativity recognises pressure as a source
of gravity just like mass, so the huge pressure resulted in powerful
gravity. "It was the sudden removal of this gravity, when
the Universe switched to a more sedate expansion at the end of
inflation, that could have produced superheavy particles,"
says Kolb. During the inflationary epoch, particles and their
antiparticles were continually conjured out of nothing as energy
was briefly borrowed from the gravitational field. However, when
inflation ended, the sudden change in gravity pulled particles
and their antiparticles farther apart, making it harder for them
to meet and annihilate each other. A similar change in gravity,
in space rather than time, causes a net production of particles
in the vicinity of a black hole, a process known as Hawking radiation.
Kolb's team says it is the strength of gravity at the end of
inflation that determines the mass of superheavy particles. Although
the strength is uncertain, they calculate that it should have
been enough to create particles about 10 trillion times heavier
than a proton. Kolb's team has also calculated the number that
would have appeared, and they say it is "just right to account
for the Universe's dark matter". If true, this could be
bad news for experimental physicists. Relatively few particles
would account for the dark matter, so the chances of detecting
them would be small.
However, Kolb says there is no reason why such particles should
not be electrically charged or feel the strong force, making
them easier to find. A positively charged particle might even
attract an electron to make a superheavy kind of hydrogen that
would be detectable. It's a long shot, Kolb admits - superheavy
particles are no more likely to make up dark matter than any
of the other exotic candidates that have been suggested, such
as "neutralinos" or "axions". "Our work
is simply a reminder that we haven't yet exhausted all the possibilities,"
he says. His team has submitted the work to Physical Review D.
Gravity and Anti-matter Scientific American 258 1988
Goldman, Hughes and Nieto (review)
The suggestion of the existence of supersymmetric partners to
the graviton, the gravitino, graviphoton etc. led to proposals
of additional weak forces moderating gravity. A similar class
of theories called metric theories depending on space-time curvature
allowed the graviton to decompose into such particles.
So far no evidence has been forthcoming for such modifications,
which suggested that the components might act differently on
mass and binding forces leading to different gravitational effects
for matter and anti-matter. This should be taken into account
in the current theory suggested below.
The Force of Darkness - generalized relativity theory
New Scientist 7 Mar 98
A MYSTERIOUS second type of gravity may help choreograph the
motion of matter in the Universe-and help save it from the singularities
nature abhors. Physicists in Britain say the force would be felt
directly only by matter with a previously unsuspected "gravitational
charge". "Such matter would interact via the new form
of gravity in addition to the familiar form between massive bodies,"
says Robin Tucker of the University of Lancaster. The existence
of gravitational charge would have profound implications for
the early Universe and for black holes. "It could prevent
the formation of a singularity, a point of infinite density,
at the beginning of time and possibly in the heart of a black
hole," says Tucker. Tucker and his colleagues had been revisiting
Einstein's theory of gravity, the general theory of relativity.
They knew that Einstein had made a certain assumption about gravity
and space-time to simplify the equations. "If you relax
this arbitrary assumption, a more general form of gravitation
can be contemplated," says Tucker. One consequence of the
general form is the existence of a new interaction coupled to
a gravitational "charge" that may be carried by some
types of matter. The charge comes in two kinds, similar to positive
and negative electrical charges. The force would be carried by
a particle that may be as heavy as a grain of sand, and would
act over distances smaller than an atom. Normal matter does not
carry this charge. But Tucker and his colleague Charles Wang
speculate that it might be carried by the invisible dark matter
which makes up most of the mass of the Universe. The force would
have been felt in the early Universe, and in any other region
of the Universe where matter is very dense. Tucker and Wang found
that if the gravitational charge has one sign, it might balance
the normal attraction of gravity with a repulsive force. This
could avoid the problem of a singularity at the beginning of
time and in black holes. Alternatively, if the gravitational
charge on dark matter has the opposite sign, the normal attraction
of gravity would be reinforced, producing a more pronounced singularity
and affecting the expansion rate of the Universe. Either way,
the presence of gravitational charge could have determined the
kind of Universe we live in. Gravitational charge could also
be having a more indirect effect on today's Universe. Although
the new interaction has a very short range, it can nevertheless
affect the shape of space-time and this would in turn affect
the motion of normal matter. "If the Sun had a small gravitational
charge, it would affect the motion of the planets," says
Tucker. "We can therefore use our knowledge of planetary
motion to set bounds on the amount of gravitational charge in
our neighbourhood." His results will appear in next month's
issue of Classical and Quantum Gravity. Marcus Chown
Could gravity explain quantum mechanics? (review) New Scientist
7 Mar 98
In an unusual Ph.D. thesis Mark Hadley has set out a theory which
interprets particles as knotted regions of space-time in which
there is a time-like loop as well as space contained within the
particle. Wormholes in space-time conceived by Misner and Wheeler
were traditionally only spatial distortions because of the causal
paradox time loops would involve, but Hadley sees these properties
as exactly those generating the known causality violations of
He notes that a particle appearing to interact with its own history
in a causality violating way could precisely create the indefinition
required to explain quantum uncertainty. He also notes that such
an object would interact with both its past and its future too
much as a jiggling rope connecting all tits boundary conditions
in a way which would make specifying its motion from the initial
conditions impossible. "Not surprisingly this changes everything"
says Hadley. "For a quantumparticle there is another end,
another unknown boundary condition in the future and not everything
is determined". The non-commutation of quantum variables
could also be explained by properties leaking into the particle's
Constant Chaos New Scientist 28 Mar 98
LIGHT from far-flung quasars is threatening to revolutionise
physics. An international team of scientists say they may have
found evidence from these distant beacons that a constant which
determines the strength of the electromagnetic force may have
been different earlier in the Universe. "It would certainly
be a major development, making obsolete much of what we know
about the Universe," says astronomer Mike Hawkins of the
Royal Observatory in Edinburgh. The constant in question is known
as the fine structure constant [e^2/hc], which depends on three
other quantities: tile charge on the electron, Planck's constant
and the speed of light. "If our results are correct, one
or more of these constants must have caried over the history
of the Universe," says Christopher Churchill in Pennsylvania
State University. Working with Churchill and John Barrow at Sussex
University, team leader John Webb and his colleagues at the University
of New South Wales in Sydney used a new technique to measure
the fine structure constant at various epochs of the Universe.
They looked at "bites" taken out of quasar spectra
by light-absorbing atoms in gas clouds between the quasars and
Earth. The difference between the wavelengths absorbed by any
two elements is sensitive to the value of the fine structure
constant at the time of absorption in each cloud. The team measured
wavelength differences to within a ten-thousandth of a nanometre
at the 10-metre Keck telescope in Hawaii. The results showed
that the fine structure constant was several parts in 100 000
smaller than today between a red shift of 1.0 and 1.4. The red
shift is a measure of how much the Universe has expanded since
the clouds emitted light, and is a rough guide to their age.
The researchers have submitted the results to Physical Review
Letters. If the variation is real, the implications are profound.
Many theories attempting to unify gravity with the other forces,
such as string theory, require extra dimensions of space-time.
We don't experience them because they are "rolled up"
smaller than an atom. In such theories, the constant that determines
the strength of the electromagnetic force depends on how tightly
the extra dimensions are rolled up. "If the fine structure
has changed with time," says Churchill, "it could be
evidence that the size of the rolled-up dimensions has changed."
In the past, several scientists, among them British physicist
Paul Dirac, have speculated that fundamental constants of physics
evolve with time. According to Churchill, such changes might
be coniiected with changes in the energy density of the vacuum.
He points out, for instance, that the speed of light depends
on the interaction between photons and the quantum vacuum which
seethes with "virtual" particles popping in and out
of existence. "if the energy density of the vacuum were
greater in the past, the speed of light would be slightly different,"
he says. Differences in the fine strructure constant could also
have affected element-building nuclear reactions in stars, which
are extremely sensitive to such physical constants. "A change
in the fine structure constant could change the rate at wliicil
stars burn their fuel and so subtly alter the entire evolution
of stars," savs Churchill. "It's hard to imagine the
full ramifications of changes to the fine structure constant,"
comments Hawkins. He says that depending on how the constant
changed with time, it might make some atoms unstable during certain
epochs, or alter the wavelength of the cosmic microwave background-the
radiation left over from the big bang fireball. "If the
variation is true, it is extremely significant," adds Tom
Kibble, a theoretical physicist at Imperial College, London.
"But I would certainly want to look for other possible explanations
of the data before accepting this one." Churchill admits
the results could be a mirage due to small calibration errors:
"We're currently working very hard to rule this out."
Einstein in free fall NS 13 Jun 98 11
A FRESH clash between Einstein's general theory of relativity
and quantum mechanics has come to light. A physicist in New Mexico
claims that quantum mechanics predicts that particles on Earth
are affected by massive objects millions of light years awav.
If he is right, one of the basic asswnptions of Einstein's theory
must be wrong. A central premise of general relativity is that
you cannot tell the difference between being in free fall towards
a massive object and being in no gravitational field at all.
Someone sitting in a capsule which is failing towards a shell
of matter would feel exactly the same as someone inside that
shell, where the gravitational forces balance out to zero. Neither
would feel themselxves pressing down on their seats. in other
words, objects are indifferent to their gravitational "potential":
how tightly bound they are to a gravitational body. "If
you look at the foundations of general relativity, it's stronglv
dependent on this notion of free fall," savs Dharam Ahluwalia,
a physicist at Los Alamos National Laboratory. But Ahluwalia
says that quantum mechanics may soon demolish the idea that objects
cannot sense their potential. Quantum mechanics has already overturned
a good deal of classical theory, such as the laws of electroma,netism.
Before quantum theory, physicists thouaht that an electron shooting
past an ideal solenoid-a tube which has maanetic fields on the
inside but not on the outside-would be unaffected by the field.
But because of the "smeariness" of real electrons,
they are affected by a field they shouldn't be able to "see".
Ahluwalia put gravity into the Schr6dinger equation, which is
normally used to describe the quantum behaviour of a particle
in different electromagnetic potentials. He found there is a
gravitational analogue of the solenoid effect: particles can
"feel" their gravitational potential. In a forthcoming
issue of Modern Physics Letters B, Ahluwalia says that this effect
would influence the way neutrinos flip from one type to another.
Scientists reported evidence for this behaviour last week (see
p 25). Ahluwalia says neutrinos with mass would "feel"
their gravitational potential, and one with a large potential
at the centre of a shell would change from one neutrino "flavour"
to another more slowly than one in free fall a large distance
away. "Personally, I believe it must be true," says
Samuel Wemer of the University of Missouri in Columbia, who is
hoping to see similar effects at work in electrons at the centre
of a tube filled with a tonne of mercury. "In principle,
it could be observed." if confirmed, the new idea would
imply there are tiny inaccuracies in some predictions of general
relativity theory. It would also suggest that distant galaxies
affect the properties of nearby particles by contributing enormous
potential. The black hole at the centre of the Milky Way and
the galaxies and dark matter that make up the "Great Attractor"
which is pulling on our Galaxy would both change how quickly
neutrinos oscillate near the Earth.
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