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Neutrino ripples spotted in space

June 17, 2005 By Mark Peplow This article courtesy of Nature News.

Universal lumpiness is imprinted in mysterious particles.

Astronomers have spotted a signature of neutrinos created just seconds after the Big Bang.

The find supports current models of the origins of our Universe, and may provide a glimpse of its birth.

The fundamental particles called neutrinos are difficult to study, because they interact so weakly with normal matter - trillions whizz straight through your body every second. But Roberto Trotta, an astrophysicist from Oxford University, UK, and his colleague Alessandro Melchiorri of the University of Rome 'La Sapienza', Italy, say that the signature of primordial neutrinos is written in the cosmic microwave background (CMB).

These microwaves are the remnants of light that shone 300,000 years after the Big Bang, when light was first free to move in a straight line without being blocked by the soupy material of the early Universe.

Researchers have found that the CMB is slightly uneven, reflecting the lumpy distribution of matter in the early Universe. Wayne Hu, a cosmologist from the University of Chicago, proposed that neutrinos should affect these ripples in the CMB1. That is what Trotta and Melchiorri have found, they report in a forthcoming paper in Physical Review Letters2.

Unpredictably lumpy

During the Big Bang, matter became patchily distributed. This was a result of matter's graininess on a small scale: subatomic particles either exist in a space or they do not, making the distribution of matter unpredictably lumpy.

As the Universe grew, its lumps expanded too, spreading matter unevenly about the cosmos. The CMB, for example, contains ripples separated by about one degree - the same size as a full Moon seen from Earth.

Trotta and Melchiorri worked on the assumption that fast-moving, energetic neutrinos in the early Universe changed the local gravity enough to smooth out some of the ripples in the CMB. The neutrinos' influence would have been minute, but potentially visible.

When they looked at the CMB on a scale of about a hundredth to a thousandth of a degree, they found less variation than expected. This fits with the prediction that neutrinos have a smoothing effect. "The fact that we can see this in the WMAP data was a big surprise," says Trotta.

Right direction

The find could help astrophysicists peer further back in time. The earliest we can see at the moment is to 300,000 years after the Big Bang. But neutrinos would have shaped the CMB from a few seconds after the Universe's birth.

Hu adds that learning about neutrinos in the early Universe and their interaction with the CMB should teach researchers about the other particles of that time. Such particles might, for example, stop neutrinos from smoothing out the CMB.

The observations aren't definitive yet. "It's not quite strong enough to call it a detection, but it goes in the right direction," says Trotta. He adds that the next set of data on the CMB, expected this year, could provide a firmer answer.

In the meantime, Hu says it's reassuring that the results are consistent with theoretical predictions. "We have been surprised in the past with missing matter and energy in earlier versions of the standard cosmological model," says Hu. "It is certainly possible that we will be surprised again."


  1. Hu W., Eisenstein D. J., Tegmark M. & White M. J. Phys. Rev. D, 59. 023512 (1999).
  2. Trotta R. & Melchiorri A. Phys. Rev. Lett., preprint available at (2004).


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