Alternative theory of gravity explains large structure formation -- without dark matter

Dec 14, 2006 feature
Alternative theory of gravity explains large structure formation -- without dark matter
The light from galaxies in the background has been warped and “arced” by the galaxy cluster Abell 1689 in the foreground, and perhaps with some help by either dark matter or a stronger type of gravity on this large scale. Image Source: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA.

In the standard theory of gravity—general relativity—dark matter plays a vital role, explaining many observations that the standard theory cannot explain by itself. But for 70 years, cosmologists have never observed dark matter, and the lack of direct observation has created skepticism about what is really out there.

Lately, some scientists have turned the question around, from “is dark matter correct?” to “is our standard theory of gravity correct?” Most recently, Fermilab scientists Scott Dodelson and former Brinson Fellow Michele Liguori demonstrated one of the first pieces of theoretical evidence that an alternative theory of gravity can explain the large scale structure of the universe.

“To definitively claim that dark matter is the answer, we need to find it,” Dodelson explained to PhysOrg.com. “We can do this in one of three ways: produce it in the lab (which might happen at Fermilab, the soon-to-start LHC, or ultimately the International Linear Collider), see a pair of dark matter particles annihilate and produce high energy photons (there are about a half dozen experiments designed to look for this), or see a dark matter particle bump a nucleus in a large underground detector (again, about 10 experiments are looking for this). Until one or more of these things happen, skeptics are still allowed. … After they happen, skeptics will become crackpots.”

Although cosmologists have never directly observed dark matter, they have many good reasons for not giving up hope. The ways that galaxies rotate and starlight bends (gravitational lensing) stray from predictions based on visible matter. Further, the formation of large cosmic structures (such as galaxies and galaxy clusters) would have required significantly large matter perturbations when the Universe was less than a million years old that simply don’t exist in a theory of general relativity before “tacking on” dark matter.

“It is extremely important to see how well a no-dark-matter cosmology does,” said Dodelson. “[In the standard theory,] we are asking the community to believe in the existence of a particle that has never been seen. We have to be damned sure that you can't explain the universe without this huge leap. Our Figure 1 [see citation below] illustrates that, in standard gravity, a no-dark-matter model does not do well at all.”

While altering the theory of gravity may seem like pulling the rug out from under a century of observations and pain-staking calculations, an alternative theory may simply be “more correct” than today’s standard theory. Just as Einstein’s theory was “more correct” than Newton’s because it improved upon the older one by noticing more specific details (e.g. extraordinary masses and speeds), a new alternative theory may only drastically change gravity at certain scales.

“Perhaps a fundamental theory of gravity which differs from general relativity on large scales can explain the observations without recourse to new, unobserved particles,” wrote Dodelson and Liguori in their study published in Physical Review Letters. “Now more than ever before, there are very good reasons to explore this idea of modifying gravity. For, the case of dark energy also hinges on the assumption that general relativity describes gravity on larges scales. Dark energy is even more difficult to explain than dark matter, so it seems almost natural to look at gravity as the culprit in both cases.”

The new theory (or groundwork for it) under investigation would be Jacob Bekenstein’s relativistic covariant theory of gravity (TeVeS), published in 2004. Bekenstein based his theory on a modified version of Newtonian theory from the early ‘80s, dependent on gravitational acceleration and called modified Newtonian dynamics (MOND) by its founder, Mordecai Milgrom.

“MOND, the original theory on which TeVeS is based, was already quite successful at explaining galactic dynamics (even better, in some cases, than the dark matter paradigm), but it failed completely at explaining other observations—gravitational lensing in particular,” explained Liguori. “For this reason, it couldn't be considered a real alternative to dark matter. Bekenstein’s theory, by generalizing MOND, retains its good features while overcoming its main problems at the same time. This makes TeVeS a much more interesting theory than MOND. It is then worthwhile (and necessary) to test TeVeS’ predictions in detail and compare them to the standard dark matter paradigm to see if TeVeS can be a viable alternative.”

Dodelson and Liguori find Bekenstein’s theory intriguing in this context because, for one, the gravitational acceleration scale in the theory is very close to that required for the observed acceleration of the Universe. The scale is also very similar to that proposed in “post hoc” theories such as dark energy. Even more interesting is the fact that the origins of Bekenstein’s theory had nothing to do with cosmic acceleration.

But the feature of Bekenstein’s theory that Dodelson and Liguori focus on most is that the theory—unlike standard general relativity—allows for fast growth of density perturbations arising from small inhomogeneities during recombination. Building on this finding from scientists Skordis et al. earlier this year, Dodelson and Liguori have found which aspect of the theory actually causes the enhanced growth—the part that may solve the cosmological structure problem.

The pair has discovered that, while Bekenstein’s theory has three functions which characterize space-time—a tensor, vector and scalar (TeVeS)—it’s the perturbations in the vector field that are key to the enhanced growth. General relativity describes space-time with only a tensor (the metric), so it does not include these vector perturbations.

“The vector field solves only the enhanced growth problem,” said Dodelson. “It does so by exploiting a little-known fact about gravity. In our solar system or galaxy, when we attack the problem of gravity, we solve the equation for the Newtonian potential. Actually, there are two potentials that characterize gravity: the one usually called the Newtonian potential and the perturbation to the curvature of space. These two potentials are almost always very nearly equal to one another, so it is not usually necessary to distinguish them.

“In the case of TeVeS, the vector field sources the difference between the two,” he continued. “As it begins to grow, the difference between the two potentials grows as well. This is ultimately what drives the overdense regions to accrete more matter than in standard general relativity. The quite remarkable thing about this growth is that Bekenstein introduced the vector field for his own completely independent reasons. As he remarked to me, ‘Sometimes theories are smarter than their creators.’"

Dodelson and Liguori see this solution to large structure formation as an important step for a gravity theory based on baryon-only matter. Other problems that their theory (or any alternative theory) will have to confront include accounting for the mismatch in galaxy clusters between mass and light. Also, the theory must conform to at least two observations: the galaxy power spectrum on large scales, and the cosmic microwave background fluctuations, which correspond to baby galaxies and galaxy clusters.

“As Scott says, until dark matter will be observed, skeptics will be allowed,” said Liguori. “Despite the many and impressive successes of the dark matter paradigm, which make it very likely to be correct, we still don't have any final and definitive answer. In light of this, it is important to keep an eye open for possible alternative explanations. Even when, after the analysis, alternative theories turn out to be wrong, the result is still important, as it strengthen the evidence for dark matter as the only possible explanation of observations.”

Citation: Dodelson, Scott and Liguori, Michele. “Can Cosmic Structure Form without Dark Matter?” Physical Review Letters 97, 231301 (2006).

By Lisa Zyga, Copyright 2006 PhysOrg.com

Explore further: Do we live in a 2-D hologram? New Fermilab experiment will test the nature of the universe

add to favorites email to friend print save as pdf

Related Stories

On the hunt for dark matter

Aug 22, 2014

New University of Adelaide Future Fellow Dr Martin White is starting a research project that has the potential to redirect the experiments of thousands of physicists around the world who are trying to identify the nature ...

Recommended for you

Awakening the potential of plasma acceleration

7 hours ago

Civil engineering has begun for the new Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) at CERN. This proof-of-principle experiment will harness the power of wakefields generated by proton ...

Magnetic memories on the right track

8 hours ago

Computer hard drives store data by writing magnetic information onto their surfaces. In the future, magnetic effects may also be used to improve active memory in computers, potentially eliminating the need ...

When an exciton acts like a hole

9 hours ago

(Phys.org) —When is an electron hole like a quasiparticle (QP)? More specifically, what happens when a single electron hole is doped into a two-dimensional quantum antiferromagnet? Quasiparticle phenomena ...

User comments : 5

Adjust slider to filter visible comments by rank

Display comments: newest first

herrrumbre
1 / 5 (1) Oct 20, 2007
Is our standard theory of gravity correct ? Correct is to question the standard
model of gravity because there is no body in the cosmos with the energy or
capacity to attract any other body.Gravity is a force that bodies suffer and not
produce.No one doubt that the universe was born into a dense and very high
degree temperature energy that was transforming in matter as temperature be
gin to reduce , so matter is a form of energy.Einstein told that matter is com
press energy so matter is denser than energy.This density difference between
matter and energy is gravity so gravity is an other form of energy.The strange
form of energy that acts like repulsive gravity is an energy in balance with the
energy responsible for gravity both are equal the only difference is that when
one attract the other repel . Both energies exist in perfect equilibrium with earth.
Looking gravity with another perception we could see two energies that exist
from the very beginning of the universe and they conform what has been called
dark energy.In the same way is easy to see dark matter detected in the largest
gravitational bound galaxies clusters in the universe with millions billions solar
masses with multimillions degree Celsius gas-six times larger -than the matter of
the galaxies cluster.They are like miniatures universes with all the universe ele
ments .
With the knowledge we have we can assume 5 % for visible matter; 30 % for
Gaseous matter detected in massive galaxy clusters;two times energies in balan
Ce with matter gives 70 % , all these quantities gives 105 %. The energy that
Acts like gravity lost 5 % in density when it compressed to give birth to mat
Ter, considering that we have exactly 100 %.
What is showing the above information is that the earth is 5 % denser than the
Energies around it, this make gravity to cause bodies go down against earth .
But at a distance 5 % larger than the earth radio all the energies are alike and
Under this condition bodies do not go down , they tend to float .

Herrumbre ^ ~~~~ Oct. 22 07
Tissa_Perera
1 / 5 (2) Mar 13, 2008
In order to account for the observed galaxy dynamics,
It was natural to invent the extra mass that was needed
to explain the kinetics of the large scale. Since the extra
mass was never observed directly it was named dark matter.

In spite of having painstakingly developed the standard
model of real fermionic and bosonic matter, they were ready
to accept the existence of a third kind of matter i.e. dark matter,
in order to account for the unusual behavior of gravity, while all
the time having known that they could not unite gravity into
the standard particle model.

Some have therefore tried the next best obvious solution to the
problem and resorted to modify the behavior of gravity at large scales.
MOND and relativistic attempts of extending MOND are more
logical but fails.

I have pursued another alternative to all of the above. I announce that
the Newtonian law of gravity is universal, but the real observed mass of
objects are virtually multiplied at very long range from the object,
effectively mimicking the so called dark matter. I declare that the secret
of the mass multiplier is the existence of a cosmic size(A few Kpc%u2019s)
4th bounded spatial dimension. The 4th dimension also gave me a bonus,
I can now explain most other problems in physics.

See my web site for a preview of things to come at:
cosmicdarkmatter.com
lba
1 / 5 (2) Apr 09, 2008
I think the origin of gravity is an oscillation of electrostatic forces. The oscillation is added to another oscillation. The oscillation is very small compared with 1.6*10-19C. you can go on www.ag0.blogspot.com for more details.
HenisDov
1 / 5 (2) Apr 06, 2009
Energetic Origin Of Mass and Gravity, Commonsensible Conception


A. A note about "new Theory of Everything Breakthrough"

"We are a group that is challenging the current paradigm in physics which is Quantum Mechanics and String Theory. There is a new Theory of Everything Breakthrough. It exposes the flaws in both Quantum Theory and String Theory. Please Help us set the physics community back on the right course and prove that Einstein was right! Visit our site The Theory of Super Relativity:
Super Relativity
http://www.superr...ity.org/

Mark Fiorentino Mar. 18, 2009"


B. My note, of a commonsensible primitive mind, about the above note

I think you'll find this link of "complementary interest" to you, even if without math, and not that you need complementary ideas...

http://www.physfo...ic=22994&st=0&#entry373127

However, "my ether" is being laid by the expanding galactic-clusters...


C. Commonsensible conception of gravity

1. According to the standard model, which describes all the forces in nature except gravity, all elementary particles were born massless. Interactions with the proposed Higgs field would slow down some of the particles and endow them with mass. Finding the Higgs %u2014 or proving it does not exist %u2014 has therefore become one of the most important quests in particle physics.

However, for a commonsensible primitive mind with a commonsensible universe represented by
E=Total[m(1 D)], this conceptual equation describes gravity. It does not explain gravity. It describes it. It applies to the whole universe and to every and all specific cases, regardless of size.

2. Thus gravity is simply another face of the total cosmic energy. Thus gravity is THE cosmic parent of phenomena such as black holes and life. It is the display of THE all-pervasive-embracive strained space texture, laid down by the expanding galactic clusters, also noticed in the expanding energy backlashes into various constructs of temporary constrained energy packages.


3. "Extrapolation of the expansion of the universe backwards in time to the early hot dense "Big Bang" phase, using general relativity, yields an infinite density and temperature at a finite time in the past.
At age 10^-35 seconds the Universe begins with a cataclysm that generates space and time, as well as all the matter and energy the Universe will ever hold."

At D=0, E was = m and both E and m were, together, all the energy and matter the Universe will ever hold. Since the onset of the cataclysm E remains constant and m diminishes as D increases.
The increase of D is the inflation, followed by expansion, of what became the galactic clusters.

At 10^-35 seconds, D in E=Total[m(1 D)] was already a fraction of a second above zero. This is when gravity started. This is what started gravity. At this instance starts the space texture, starts the straining of the space texture, and starts the "space texture memory", gravity, that will eventually overcome expansion and initiate re-impansion back to singularity.


D. Commonsensible conception of the forces other than gravity

The forces other than gravity are, commonsensibly, forces involved in conjunction with evolution:
http://royalsocie...?id=4770

The farthest we go in reductionism in Everything, including in Life, we shall still end up with wholism, until we arrive at energy. Energy is the base element of everything and of all in the universe. At the beginning was the energy singularity, at the end will be near zero mass and an infinite dispersion of the beginning energy, and in-between, the universe undergoes continuous evolution consisting of myriad energy-to-energy and energy-to-mass-to-energy transformations.

The universe, and everything in it, are continuously evolving, and all the evolutions are intertwined.


Dov Henis
(Comments From The 22nd Century)
Life's Manifest
http://www.the-sc...page#578
EVOLUTION Beyond Darwin 200
http://www.physfo...ic=14988&st=405&#entry396201
http://www.the-sc...age#1407
HenisDov
1 / 5 (1) Apr 15, 2009
E. PS: On Evolution of Cosmic Energy And Mass

As mass is just another face of energy it is commonsensible to regard not only life, but mass in general, as a format of temporarily constrained energy.

It therefore ensues that whereas the expanding cosmic constructs, the galaxies clusters, are - overall - continuously converting "their share" of original pre-inflation mass back to energy, the overall evolution within them, within the clusters, is in the opposite direction, temporarily constrained energy packages are precariuosly forming and "doing best" to survive as long as "possible"...


Dov Henis
(Comments From The 22nd Century)