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FQXI ARTICLE

September 26, 2017

Out of the Darkness

As dark matter, dark energy, and general relativity slug it out against rival theories, Glenn Starkman hopes to pick out the winner.

FQXi Awardees: Glenn Starkman

March 27, 2009

GLENN STARKMAN

Case Western Reserve University

“The question is: Are we able to test these alternatives to general relativity?” says

You could say that this is the $64,000 question. Or rather the $64,760 question, since that is the size of the grant that Starkman has been awarded by the Foundational Questions Institute to investigate the problem.

Starkman didn’t always see himself as a physicist. Born in Toronto, Canada, his initial passion was neuroscience, but that gave way to a lasting fascination with how the universe worked, thanks to a mathematics professor at a local university. “I studied with Professor Moshe Shimrat just about every week from 7th grade until he died near the end of my high school career,” remembers Starkman. “He showed me the beauty of the abstract, while encouraging my interest in the measurable.”

A PhD at Stanford University with Savas Dimopoulos focused his curiosity further on the universe. And the “interest in the measurable” that Professor Shimrat instilled in him all those years ago never dimmed, leading Starkman to today investigate how to test general relativity and its rivals.

He showed me the

beauty of the abstract,

while encouraging my

interest in the

immeasurable.

beauty of the abstract,

while encouraging my

interest in the

immeasurable.

- Glenn Starkman on Moshe Shimrat

Traditionally, the strongest evidence given for dark matter is that galaxies are rotating much faster than expected. Assuming that they are glued together by the gravity of the visible matter they contain, astronomers calculated that these galaxies are spinning so quickly they should have been torn apart long ago because there simply isn’t enough matter to hold them together.

The best solution to the puzzle seemed to be that the galaxies also contain vast amounts of unseen mass or dark matter, which provides the extra gravitational tug to keep everything in place. Astronomers were thus able to hold on to their theory of gravity, but at a cost: They had to accept that the majority of matter in the universe is in some unknown, invisible, form.

BATTLEFIELD GALACTICA

Which theory best explains

what happens when galaxies collide?

Credit: NASA

The third worry for Starkman revolves around another burst of cosmic expansion, known as inflation, when the early universe went through an exponential growth spurt. Cosmologists need inflation to explain why the universe looks pretty much the same in every direction. Trouble is, general relativity can’t explain why inflation happened based on the currently observed contents of the universe; so physicists are left positing the existence of some enigmatic field—an early form of dark energy—that caused a similar accelerated expansion in the early universe.

Pretenders to the Throne

But there are alternatives to bolting on new entities like dark matter, dark energy, and the field driving inflation. “There has been longstanding interest in modifying general relativity as an alternative to repeatedly adding new forms of energy to the universe,” says Starkman.

The idea of modifying gravity was introduced in the mid-1980s. Mordehai Milgrom proposed a theory known as MOND (Modified Newtonian Dynamics), which tweaked the laws of gravity in different ranges in order to explain the weird galaxy-rotation observations.

We’ve been forced to add

dark matter, dark

energy and the field

that drives inflation.

Many consider that

contrived.

dark matter, dark

energy and the field

that drives inflation.

Many consider that

contrived.

- Glenn Starkman

Unfortunately, while MOND sounded great in principle, the idea hadn’t been developed well enough to be tested or to make predictions in the way that general relativity had. “Until then, you couldn’t really do cosmology or hope to understand how (complicated) systems behaved,” explains Starkman.

The dam began to break in 2004 with the arrival of Tensor-Vector-Scalar (TeVeS) theory, developed by Jacob Bekenstein at the Hebrew University of Jerusalem, Israel. TeVeS not only reproduced MOND’s predictions but also satisfied the principles of relativity.

Since then, others have built on Bekenstein’s success; Starkman and his collaborators recently introduced Generalized Einstein Aether, a class of theories that both generalize and simplify TeVeS.

Testing Times

OK, so now cosmologists have a bunch of theories that can take on general relativity. But how do you choose between them? One particular model of modified gravity, DGP—named after its proposers Dvali, Gabadadze and Porati—set Starkman thinking about how to test all these new varieties.

SHEDDING LIGHT ON DARK ENERGY

Supernovae told us the

expansion of the universe

is accelerating. But why?

Credit: Chandra X-Ray Observatory

But intriguingly, their calculations also showed these effects were not specific to DGP. Rather, they were general features of a class of theories that obey something known as

Birkhoff’s theorem is a little known but vital conjecture that, loosely phrased, says the gravity inside a region is independent of the matter outside it. It greatly simplifies calculations in general relativity; without it, to work out the acceleration of a body at one place you’d need to know the position of all the matter in the universe—a complex undertaking, indeed.

None of the well-developed alternatives to general relativity proposed to date obey Birkhoff’s Theorem. So, asks Starkman, can we actually compute reliably with them? And if not, then what do we do?

Starkman is using his $64,760 FQXi grant to find out the answers. If reliable calculations can be done, then physicists can test these modified gravity models against future observations. But if not, things get awkward.

Glenn Starkman is always

willing to explore

questions that may be

outside the mainstream.

willing to explore

questions that may be

outside the mainstream.

- Lawrence Krauss

Although Starkman is only at the start of this journey, he is looking forward to digging deeper. Intriguingly, there’s an exception that doesn’t flout Birkhoff. Despite its other flaws, MOND—the granddaddy of modified gravity theories—maintains Birkhoff’s theorem and Starkman is keen to understand what this implies.

In the meantime, Starkman’s peers have high hopes for this latest research strand.

“Glenn Starkman is a creative physicist who is always willing to explore questions that may be outside of the mainstream,” says Lawrence Krauss at Arizona State University, Tempe. “The question is interesting and may be relevant to understanding dark energy.”

Tom Zlosnik at the Perimeter Institute in Waterloo, Ontario, agrees. “The research questions the validity of a deep assumption in cosmology,” he says. “The next genuine advance in physics may involve a dramatic development in our conception of the universe. Perhaps this stuff might be a step towards that.”

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DOUGLAS W LIPP wrote on October 3, 2010

Coney Island Green - continued - see all posts

Get This - "Another Link"

From the Wiki - Cosmological Constant site:

The cosmological constant Λ appears in Einstein's modified field equation in the form of

ADD Field Equation Here as it would not "cut and paste" see Wiki

where R and g pertain to the structure of spacetime, T pertains to matter and energy (thought of as affecting that structure), and G and c are conversion factors that arise from...

Coney Island Green - continued - see all posts

Get This - "Another Link"

From the Wiki - Cosmological Constant site:

The cosmological constant Λ appears in Einstein's modified field equation in the form of

ADD Field Equation Here as it would not "cut and paste" see Wiki

where R and g pertain to the structure of spacetime, T pertains to matter and energy (thought of as affecting that structure), and G and c are conversion factors that arise from...

DOUGLAS W LIPP wrote on September 14, 2010

To be pieced together with CIG Theory :

The dissapearance of length along the x-axis only is consistent with the fact that at the rate "c", only the forefront of the matter in motion observes that motion, while that matter behind it moves relative to the particles intimately close by - i. e. relative to their nearby matter friends, they are not in motion. Only the x-axis sees the rate "c". The matter chunks directly behind the forefront x-axis wait their turn to see that rate (unitil...

To be pieced together with CIG Theory :

The dissapearance of length along the x-axis only is consistent with the fact that at the rate "c", only the forefront of the matter in motion observes that motion, while that matter behind it moves relative to the particles intimately close by - i. e. relative to their nearby matter friends, they are not in motion. Only the x-axis sees the rate "c". The matter chunks directly behind the forefront x-axis wait their turn to see that rate (unitil...

DOV HENIS wrote on August 25, 2010

Natural Selection Derives From Cosmic Expansion

Two suggested editorial items:

I.

Origin And Nature Of Natural Selection

Update Concepts And Comprehension

Life is another mass format.

All mass formats are subject to natural selection.

Natural selection is delaying conversion of mass to energy fueling cosmic expansion.

Cosmic expansion is reconversion of all mass to energy.

Natural Selection Updated 2010

Beyond Historical...

Natural Selection Derives From Cosmic Expansion

Two suggested editorial items:

I.

Origin And Nature Of Natural Selection

Update Concepts And Comprehension

Life is another mass format.

All mass formats are subject to natural selection.

Natural selection is delaying conversion of mass to energy fueling cosmic expansion.

Cosmic expansion is reconversion of all mass to energy.

Natural Selection Updated 2010

Beyond Historical...

read all article comments