Researchers interpret asymmetry in early universe

From the NASA/WMAP Science Team:
"The expansion of the universe over most of its history has been relatively gradual. The notion that a rapid period of 'inflation' preceded the big bang expansion was first put forth 25 years ago. The new WMAP observations favor specific inflation scenarios over other long-held ideas."

December 17, 2008

Provided by Caltech, Pasadena, California

The Big Bang is widely considered to have obliterated any trace of what came before. Now, astrophysicists at the California Institute of Technology (Caltech) think that their new theoretical interpretation of an imprint from the earliest stages of the universe may also shed light on what came before.

"It's no longer completely crazy to ask what happened before the Big Bang," said Marc Kamionkowski, Caltech's Robinson Professor of Theoretical Physics and Astrophysics. Kamionkowski joined graduate student Adrienne Erickcek and senior research associate in physics Sean Carroll to propose a mathematical model explaining an anomaly in what is supposed to be a universe of uniformly distributed radiation and matter.

Their investigations turn on a phenomenon called inflation, first proposed in 1980, which posits that space expanded exponentially in the instant following the Big Bang. "Inflation starts the universe with a blank slate," Erickcek said. The hiccup in inflation, however, is that the universe is not as uniform as the simplest form of the theory predicts it to be. Some parts of it are more intensely varied than others.

Until recently, measurements of the Cosmic Microwave Background (CMB) radiation, a form of electromagnetic radiation that permeated the universe 400,000 years after the Big Bang, were consistent with inflation — miniscule fluctuations in the CMB seemed to be the same everywhere. But a few years ago, some researchers, including a group led by Krzysztof Gorski of NASA's Jet Propulsion Laboratory (JPL), which is managed by Caltech, scrutinized data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP). They discovered that the amplitude of fluctuations in the CMB is not the same in all directions.

"If your eyes measured radio frequency, you'd see the entire sky glowing. This is what WMAP sees," Kamionkowksi said. WMAP depicts the CMB as an afterglow of light from shortly after the Big Bang that has decayed to microwave radiation as the universe expanded over the past 13.7 billion years. The probe also reveals more pronounced mottling — deviations from the average value — in the CMB in half of the sky than the other.

"It's a certified anomaly," Kamionkowski said. "But since inflation seems to do so well with everything else, it seems premature to discard the theory." Instead, the team worked with the theory in their math addressing the asymmetry.

They started by testing whether the value of a single energy field thought to have driven inflation, called the inflaton, was different on one side of the universe than the other. It didn't work. They found that if they changed the mean value of the inflaton, the mean temperature and amplitude of energy variations in space also changed. So they explored a second energy field, called the curvaton, which had been previously proposed to give rise to the fluctuations observed in the CMB. They introduced a perturbation to the curvaton field that turns out to affect only how temperature varies from point to point through space, while preserving its average value.

The new model predicts more cold than hot spots in the CMB, Kamionkowski said. Erickcek adds that this prediction will be tested by the Planck satellite, an international mission led by the European Space Agency with significant contributions from NASA, scheduled to launch in April 2009.

For Erickcek, the team's findings hold the key to understanding more about inflation. "Inflation is a description of how the universe expanded," she said. "Its predictions have been verified, but what drove it and how long did it last? This is a way to look at what happened during inflation, which has a lot of blanks waiting to be filled in."

But the perturbation that the researchers introduced may also offer the first glimpse at what came before the Big Bang, because it could be an imprint inherited from the time before inflation. "All of that stuff is hidden by a veil, observationally," Kamionkowski said. "If our model holds up, we may have a chance to see beyond this veil."

A theory of everything

This image shows the how the Big Bang formed our universe. Astronomy: Roen Kelly

January 30, 2008

Provided by Urbana-Champaign

Ancient light absorbed by neutral hydrogen atoms could be used to test certain predictions of string theory, say cosmologists at the University of Illinois. Making the measurements, however, would require a gigantic array of radio telescopes to be built on Earth, in space or on the Moon.

String theory, a theory whose fundamental building blocks are tiny one-dimensional filaments called strings, is the leading contender for a "theory of everything." Such a theory would unify all four fundamental forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity). But finding ways to test string theory has been difficult.

Now, cosmologists at the U of I say absorption features in the 21-centimeter spectrum of neutral hydrogen atoms could be used for such a test.

"High-redshift, 21-centimeter observations provide a rare observational window in which to test string theory, constrain its parameters and show whether or not it makes sense to embed a type of inflation, called brane inflation, into string theory," says Benjamin Wandelt, a professor of physics and of astronomy at the U of I.

"If we embed brane inflation into string theory, a network of cosmic strings is predicted to form," Wandelt says. "We can test this prediction by looking for the impact this cosmic string network would have on the density of neutral hydrogen in the universe."

About 400,000 years after the Big Bang, the universe consisted of a thick shell of neutral hydrogen atoms (each composed of a single proton orbited by a single electron) illuminated by what became known as the cosmic microwave background.

Because neutral hydrogen atoms readily absorb electromagnetic radiation with a wavelength of 21 centimeters, the cosmic microwave background carries a signature of density perturbations in the hydrogen shell, which should be observable today, Wandelt says.

Cosmic strings are filaments of infinite length. Their composition can be loosely compared to the boundaries of ice crystals in frozen water. When water in a bowl begins to freeze, ice crystals will grow at different points in the bowl, with random orientations. When the ice crystals meet, they usually will not be aligned to one another. The boundary between two such misaligned crystals is called a discontinuity or a defect.

Cosmic strings are defects in space. A network of strings is predicted by string theory (and also by other supersymmetric theories known as Grand Unified Theories, which aspire to unify all known forces of nature except gravity) to have been produced in the early universe, but has not been detected so far.

Cosmic strings produce characteristic fluctuations in the gas density through which they move, a signature of which will be imprinted on the 21-centimeter radiation.

The cosmic string network predicted to occur with brane inflation could be tested by looking for the corresponding fluctuations in the 21-centimeter radiation. Like the cosmic microwave background, the cosmological 21-centimeter radiation has been stretched as the universe has expanded. Today, this relic radiation has a wavelength closer to 21 meters, putting it in the long-wavelength radio portion of the electromagnetic spectrum.

To precisely measure perturbations in the spectra would require an array of radio telescopes with a collective area of more than 1,000 square kilometers. Such an array could be built using current technology, Wandelt says, but would be prohibitively expensive.

If such an enormous array were eventually constructed, measurements of perturbations in the density of neutral hydrogen atoms could also reveal the value of string tension, a fundamental parameter in string theory, Wandelt says. "And that would tell us about the energy scale at which quantum gravity begins to become important."

Universal blackout

photo:The central regions of this galaxy contain primarily older, yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young, blue stars. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight. AURA / STScI / NASA

August 19, 2003

After the lights went out in the northeastern United States and Canada last week, it took only about a day for power to be fully restored to the thousands of communities that succumbed to the darkness. Meanwhile, astronomers have noticed another, more gradual blackout is spreading through the universe, but it appears that there's no hope that the lights will ever shine so brightly again in its countless galactic neighborhoods.

Ben Panter and Alan Heavens from the Institute for Astronomy at Edinburgh University recently teamed with University of Pennsylvania colleague Raul Jimenez to sneak a peek at a few nearby galaxies — 37,752 of them, to be exact. The trio used this data, freshly culled from the ongoing Sloan Digital Sky Survey (SDSS), to create the first complete histories of star formation and chemical composition for such a voluminous and diverse population.

The results, appearing in this week's Monthly Notices of the Royal Astronomical Society, confirm longstanding notions that the universe is past its birthing prime and already well on the way toward a dark and frigid future.

Stars — from brilliant and short-lived blue supergiants to steady, slow-burning red dwarfs — typically form within clouds of molecular gas dispersed throughout galaxies of all shapes and sizes. The amalgam of light from any particular galaxy represents a luminous inventory of every star within. However, sifting through galaxy light and attributing trends to single classes of stars and their formation rates over time can be exceedingly difficult.

With uber-sets of data like this one from the Sloan survey, the problem becomes even more problematic unless novel methods are employed. For each galaxy spectrum in the SDSS database, there are nearly 4,000 measurements in 25 dimensions. Straightforward analysis of one spectrum could take two years on a high-end PC workstation running non-stop.Rather than loading 40,000 galaxies into the computer for one hellish, 80,000-year all-nighter, Heavens and his partners required a robust method that would simultaneously amputate analysis time and increase accuracy without losing a drop of valuable data. In 2000, Heavens and Jimenez developed what came to be known as the MOPED (Multiple Optimized Parameter Estimation and Data) compression technique, which combines (or compresses) data into manageable blocks in a simple, yet meaningful, manner.

According to Heavens, his team's analysis of the SDSS data indicate that the stellar birth rate of the 14-billion-year-old universe peaked roughly six billion years ago, around the time that the sun was born. Ever since, star formation has been on a downward slope of stellar menopause. In addition, the researchers found that the chemical distribution of star-forming gas peaked roughly three billion years ago.

Astronomers have long pointed to the fact that many galaxies have a reddish appearance — indicating a paucity of young, hot, blue stars and a preponderance of older, redder stars — as probable evidence that the star-formation era is coming to a close. The latest results from Heavens and his colleagues are the first to confirm such inklings without the aid of any simplifying assumptions.

With galaxies' stellar populations continuing to age and available metal-rich star-forming gas on the decline, the universe seems doomed to slip gradually into a universal blackout.

Astronomers Simulate the First Stars Formed After the Big Bang

Date: Friday, August 01, 2008

photo: Artist concept of the first stars.
Credit: Harvard Smithsonian CfA

What were the first stars like that formed shortly after the Big Bang? We don't know much about the conditions of the early universe 13 billion years ago, but a new computer simulation provides the most detailed picture yet of the first stars and how they came into existence. The composition of the early universe was quite different from that of today, said Dr. Naoki Yoshida, Nagoya University in Nagoya, Japan and Dr. Lars Hernquist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. An article that will be published to the August 1 journal Science describes their findings from the computer model that simulates the early days of the universe, the "cosmic dark ages," where the physics governing the universe were somewhat simpler. The astronomers believe small, simple protostars formed, which eventually became massive, but short-lived stars.

According to their simulations, gravity acted on minute density variations in matter, gases, and the mysterious "dark matter" of the universe after the Big Bang in order to form the early stages of a star called a protostar. With a mass of just one percent of our Sun, Dr. Yoshida's simulation also shows that the protostar would likely evolve into a massive star capable of synthesizing heavy elements, not just in later generations of stars, but soon after the Big Bang. These stars would have been up to one hundred times as massive as our Sun and would have burned for no more than one million years. "This general picture of star formation, and the ability to compare how stellar objects form in different time periods and regions of the universe, will eventually allow investigation in the origins of life and planets," said Hernquist.

"The abundance of elements in the Universe has increased as stars have accumulated," he says, "and the formation and destruction of stars continues to spread these elements further across the Universe. So when you think about it, all of the elements in our bodies originally formed from nuclear reactions in the centers of stars, long ago."

The goal of their research is to be able to figure out how the primordial stars formed, as well as predicting the mass and properties of the first stars of the universe. The researchers hope to eventually extend this simulation to the point of nuclear reaction initiation – when a stellar object becomes a true star. But that's the point where the physics becomes much more complicated, and the researchers say they'll need more computational resources to simulate that process.