Instead of trying to develop theories of quantum gravity, some physicists are looking at the existing law of gravity and trying to find specific modifications that will make it work to explain the current mysteries of cosmology. These efforts are largely motivated by attempts to find alternatives to the cosmological theories of inflation, dark matter, or dark energy.
These approaches don’t necessarily resolve the conflicts between quantum physics and general relativity, but in many cases they make the conflict less important. The approaches tend to result in singularities and infinities falling out of the theories, so there just isn’t as much need for a theory of quantum gravity.
Doubly special relativity (DSR): Twice as many limits as ordinary relativity
One intriguing approach is doubly special relativity or deformed special relativity (abbreviated as DSR either way you slice it), originally developed by Giovanni Amelino-Camelia. In special relativity, the speed of light is constant for all observers. In DSR theories, all observers also agree on one other thing — the distance of the Planck length.
In Einstein’s relativity, the constancy of the speed of light places an upper speed limit on everything in the universe. In DSR theories, the Planck length represents a lower limit on distance. Nothing can go faster than the speed of light, and nothing can be smaller than a Planck length. The principles of DSR may be applicable to various quantum gravity models, such as loop quantum gravity, though so far there’s no proof for it.
Modified Newtonian dynamics (MOND): Disregarding dark matter
Some physicists aren’t comfortable with the idea of dark matter and have proposed alternative explanations to resolve the problems that make physicists believe dark matter exists. One of these explanations, which involves looking at gravity in a new way on large scales, is called modified Newtonian dynamics (MOND).
The basic premise of MOND is that at low values, the force of gravity doesn’t follow the rules laid out by Newton more than 300 years ago. The relationship between force and acceleration in these cases may turn out not to be exactly linear, and MOND predicts a relationship that will yield the results observed based on only the visible mass for galaxies.
In Newtonian mechanics (or, for that matter, in general relativity, which reduces to Newtonian mechanics at this scale), the gravitational relationships between objects are precisely defined based on their masses and the distance between them. When the amount of visible matter for galaxies is put into these equations, physicists get answers that show that the visible matter just doesn’t produce enough gravity to hold the galaxies together. In fact, according to Newtonian mechanics, the outer edges of the galaxies should be rotating much faster, causing the stars farther out to fly away from the galaxy.
Because scientists know the distances involved, the assumption is that somehow the amount of matter has been underestimated. A natural response to this (and the one that most physicists have adopted) is that there must be some other sort of matter that isn’t visible to us: dark matter.
There is one other alternative — the distances and matter are correct, but the relationship between them is incorrect. MOND was proposed by Israeli physicist Mordehai Milgrom in 1981 as a means of explaining the galactic behavior without resorting to dark matter.
Most physicists have ruled MOND out, because the dark matter theories seem to fit the facts more closely. Milgrom, however, has not given up, and in 2009 he made predictions about slight variations in the path of planets based on his MOND calculations. It remains to be seen if these variations will be observed.
Variable speed of light (VSL): Light used to travel even faster
In two separate efforts, physicists have developed a system where the speed of light actually would not be constant, as a means of explaining the horizon problem without the need of inflation. The earliest system of the variable speed of light (VSL) was proposed by John Moffat (who later incorporated the idea into his modified gravity theory), and a later system was developed by Joao Magueijo and Andreas Albrecht.
Proving dark matter wrong?
In August 2008, a group of astrophysicists published a paper called “A Direct Empirical Proof of the Existence of Dark Matter.” The “proof” they speak of came from an impact between two galaxy clusters. Using NASA’s Chandra X-Ray Observatory, they were able to see gravitational lensing (the gravity of the collision caused light to bend, kind of how light bends when it passes through a lens), which let them determine the center of the collision. The center of the collision did not match the center of the visible matter. In other words, the center of gravity and the center of visible matter didn’t match. That’s pretty conclusive evidence for there being nonvisible matter, right?
In the world of theoretical physics, nothing is quite that easy these days. By September, physicist John Moffat and others were beginning to cast doubt on whether dark matter was the only explanation. Using his own modified gravity (MOG) theory, Moffat performed a calculation on a simplified 1-dimensional version of the collision.
Most physicists accept the NASA findings, including more recent findings from WMAP and other observations, as conclusive evidence that dark matter exists. But there remain those who are unconvinced and search for other explanations.
The horizon problem is based on the idea that distant regions of the universe couldn’t communicate their temperatures because they are so far apart light hasn’t had time to get from one to the other. The solution proposed by inflation theory is that the regions were once much closer together, so they could communicate (see topic 9 for more on this).
In VSL theories, another alternative is proposed: The two regions could communicate because light traveled faster in the past than it does now.
Moffat proposed his VSL model in 1992, allowing for the speed of light in the early universe to be very large — about 100,000 trillion trillion times the current values. This would allow for all regions of the observable universe to easily communicate with each other.
To get this to work out, Moffat had to make a conjecture that the Lorentz invariance — the basic symmetry of special relativity — was somehow spontaneously broken in the early universe. Moffat’s prediction results in a period of rapid heat transfer throughout the universe that results in the same effects as an inflationary model.
In 1998, physicist Joao Magueijo came up with a similar theory, in collaboration with Aldreas Albrecht. Their approach, developed without any knowledge of Moffat’s work, was very similar — which they acknowledged upon learning of it. This work was published a bit more prominently than Moffat’s (largely because they were more stubborn about pursuing
publication in the prestigious Physical Review D, which had rejected Moffat’s earlier paper). This later work has inspired others, such as Cambridge physicist John Barrow, to begin investigating this idea.
One piece of support for VSL approaches is that recent research by John Webb and others has indicated that the fine-structure constant may not have always been constant. The fine-structure constant is a ratio made up from Planck’s constant, the charge on the electron, and the speed of light. It’s a value that shows up in some physical equations. If the fine-structure constant has changed over time, then at least one of these values (and possibly more than one) has also been changing.
The spectral lines emitted by atoms are defined by Planck’s constant. Scientists know from observations that these spectral lines haven’t changed, so it’s unlikely that Planck’s constant has changed. (Thanks to John Moffat for clearing that up.) Still, any change in the fine-structure constant could be explained by varying either the speed of light or the electron charge (or both).
Physicists Elias Kiritsis and Stephon Alexander independently developed VSL models that could be incorporated into string theory, and Alexander later worked with Magueijo on refining these concepts (even though Magueijo is critical of string theory’s lack of contact with experiment).
These proposals are intriguing, but the physics community in general remains committed to the inflation model. Both VSL and inflation require some strange behavior in the early moments of the universe, but it’s unclear that inflation is inherently more realistic than VSL. It’s possible that further evidence of varying constants will ultimately lead to support of VSL over inflation, but that day seems a long way off, if it ever happens.
Modified gravity (MOG): The bigger the distance, the greater the gravity
John Moffat’s work in alternative gravity has resulted in his modified gravity (MOG) theories, in which the force of gravity increases over distance, and also the introduction of a new repulsive force at even larger distances. Moffat’s MOG actually consists of three different theories that he has developed over the span of three decades, trying to make them simpler and more elegant and more accessible for other physicists to work on.
This work began in 1979, when Moffat developed nonsymmetric gravitational theory (NGT), which extended work that Einstein tried to apply to create a unified field theory in the context of a non-Riemannian geometry. The work had failed to unify gravity and electromagnetics, like Einstein wanted, but Moffat believed that it could be used to generalize relativity itself.
Over the years, NGT ultimately proved inconclusive. It was possible that its predictions (such as the idea that the sun deviated from a perfectly spherical shape) was incorrect or that the deviation was too small to be observed.
In 2003, Moffat developed an alternative with the unwieldy name Metric-Skew-Tensor Gravity (MSTG). This was a symmetric theory (easier to deal with), which included a “skew” field for the nonsymmetric part. This new field was, in fact, a fundamentally new force — a fifth fundamental force in the universe.
Unfortunately, this theory remained too mathematically complicated in the eyes of many, so in 2004 Moffat developed Scalar-Tensor-Vector Gravity (STVG). In STVG, Moffat again had a fifth force resulting from a vector field called a phion field. The phion particle was the gauge boson that carried the fifth force in the theory.
According to Moffat, all three theories give essentially the same results for weak gravity fields, like those we normally observe. The strong gravitational fields needed to distinguish the theories are the ones that always give scientists problems and have motivated the search for quantum gravity theories in the first place. They can be found at the moment of the big bang or during the stellar collapses that may cause black holes.
There are indications that STVG yields results very similar to Milgrom’s MOND theory (refer to the earlier section “Modified Newtonian dynamics (MOND): Disregarding dark matter” for a fuller explanation of MOND). Moffat has proposed that MOG may actually explain dark matter and dark energy, and that black holes may not actually exist in nature.
While these implications are amazing, the work is still in the very preliminary stages, and it will likely be years before it (or any of the other theories) is developed enough to have any hope of seriously competing with the entrenched viewpoints.
