(Phys.org) —Among the four fundamental forces of nature, only gravity has not had a basic unit, or quanta, detected. Physicists expect that gravitational force is transmitted by an elementary particle called a graviton, just as the electromagnetic force is carried by the photon.

While there are deep theoretical reasons why gravitons should exist, detecting them may be physically impossible on Earth.

For example, the conventional way of measuring gravitational forces – by bouncing light off a set of mirrors to measure tiny shifts in their separation – would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they'd collapse and form a black hole.

Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of – in this case the universe – to search for the telltale effects of gravitons. That is what two physicists are proposing.

In the paper, "Using cosmology to establish the quantization of gravity," published in *Physical Review D* (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early universe.

"This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons," Krauss said. "Moreover, what we find most remarkable is that the universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth."

It is generally believed that in the first fraction of a second after the Big Bang, the universe underwent rapid and dramatic growth during a period called "inflation." If gravitons exist, they would be generated as "quantum fluctuations" during inflation.

Ultimately, these would evolve, as the universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency's Planck satellite are searching for this "imprint" of inflation in the polarization of the CMB.

Krauss said his and Wilczek's paper combines what already is known with some new wrinkles.

"While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new," he said. "Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future."

Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck's constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon.

This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint.

"I'm delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity," said Wilczek.

"It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background," Krauss added.

**Explore further:**
Probing the dark side of the universe

**More information:**
"Using Cosmology to Establish the Quantization of Gravity" Lawrence M. Krauss, Frank Wilczek. *Phys. Rev. D* 89, 047501 (2014) DOI: 10.1103/PhysRevD.89.047501 , arXiv:1309.5343

## Moebius

## Andrew Palfreyman

## Bonia

Mar 04, 2014## axemaster

## Dr_toad

Mar 04, 2014## Bonia

Mar 04, 2014## Whydening Gyre

## Whydening Gyre

Silly rabbit - everything in the universe is a quanta when looked at as a single :object". Including the entire universe. any unit contains all possible states within itself - even the ones we haven't seen, yet.

## TimChase

I can see why you might think so. After all, general relativity eliminates gravity as a force and replaces it with curved spacetime, and if there is no force, it would seem that there is no need for any force carrier.

However, there also exists the Kaluza-Klein theory that unites electromagnetism and gravitation in a five dimensional theory where electromagnetism is due to how the fifth dimension interacts with the other four dimensions. The authors of this approach showed that Maxwell's equations naturally fall out of this approach.

Nevertheless, we know that the photon exists, and that it is the carrier of electromagnetic force.

## TimChase

Likewise, while Newton's gravitational theory is most easily expressed in terms of the language of force within a 3+1 dimensional space and time, it can also be expressed in terms of a four dimensional spacetime where the gravitational force is replaced by curvature between each of the spatial dimensions and the dimension of time. In this approach no curvature exists between the dimensions of space, however, and thus space itself remains flat.

The two descriptions are equivalent, however, the mathematics with which one performs calculations in the traditional approach is far simpler, and thus physicists prefer to express Newton's theory in terms of the flat 3+1 space and time. Similarly, it is possible to express general relativity in terms of a tensorial force existing in flat spacetime, but the mathematics behind the traditional curved spacetime approach is far simpler, and thus its language is preferred.

## TimChase

Finally, it should be noted that string theory is itself a curved spacetime theory in which additional dimensions are rolled up, finite, but microscopic. In string theory particles exist as excitations of the the strings.

Here are some papers that may be of interest:

Kaluza-Klein Gravity

http://arxiv.org/.../9805018

String Theory In Curved Space-Time

http://arxiv.org/.../9605007

Curved-spacetime graviton vertex operator in string theory

http://journals.a....46.3465

## Rimino

Mar 05, 2014## Rimino

Mar 05, 2014## Rimino

Mar 05, 2014## vlaaing peerd

Yes that is indeed quite an obstacle, since you know...reality and that kind of stuff.

## antialias_physorg

Pshaw...Reality is overrated. For some people the universe revolves very handily about the center of their own skull*. For them there's no need to check anything as banal as 'reality' to know what's right.

*(because that's where the biggest black hole is situated)

## Rimino

Mar 05, 2014## Shitead

If this is true it de facto requires that the speed of gravity be the speed of light squared. Cool!

## Whydening Gyre

I stated this in one of these dang articles.... Over 50, so I don't remember which one...

## baudrunner