Sunday, 26 August 2007

What Powered the Big Bang?

Appearance of universe approximately 400,000 years after the Big Bang
Watch a video or read the story of the history of the universe. [+ more]

reposted from Nasa

The night sky presents the viewer with a picture of a calm and unchanging Universe. Therefore, the discovery by Edwin Hubble, in 1929, that the Universe is in fact expanding at an enormous speed, was a revolutionary one.

Hubble noted that galaxies outside our own Milky Way were all moving away from us, each at a speed proportional to its distance away from us. Most importantly, this meant that there must have been an instant in time (now known to be about 14 billion years ago) when the entire Universe was contained in a single point in space. The Universe must have been born in this single violent event which came to be known as the "Big Bang."

Astronomers combine mathematical models with observations to develop workable theories of how the Universe came to be. The mathematical underpinning of the Big Bang included Albert Einstein's theory of general relativity, along with standard theories of fundamental particles. Today NASA spacecraft such as the Hubble Space Telescope and the Spitzer Space Telescope continue Hubble's work to measure the expansion of the Universe. In addition, the observational evidence for the details of the Big Bang now includes:

  • Background Radiation
    According to the theories of physics, one second after the Big Bang, the temperature of the Universe was roughly 10 billion degrees and was filled with a sea of neutrons, protons, electrons, anti-electrons (positrons), photons and neutrinos. As the universe cooled, the neutrons either decayed into protons and electrons or combined with protons to make deuterium (an isotope of hydrogen). Then, as the Universe continued to cool, electrons were combined with nuclei to form neutral atoms. Before this "recombination," the Universe was opaque, because the free electrons caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds. But when the free electrons were removed to form neutral atoms, the Universe suddenly became transparent. Those same photons-the afterglow of the Big Bang known as cosmic background radiation - are what we can observe today with the Wilkinson Microware Anisotropy Probe (WMAP).

  • Abundance of Elements
    If the Big Bang model is correct, the proportion of helium in the Universe should be approximately 24%. And that is just what observers have discovered.

As the years after Hubble went on, the picture of the Big Bang got clearer and clearer. Problems arose, but solutions were found.

But then something unexpected happened. In 1996, observations of very distant supernovae required a shocking change in picture. To understand this change, one must realize that since the expanding Universe was discovered, one thing was clear - that the matter of the Universe would slow down the rate of expansion. Mass creates gravity, gravity pulls on everything, the pulling must slow the expansion down. But what the supernovae observations showed was that the Universe's expansion is NOT slowing down, it is accelerating.
Something, not like matter and not like ordinary energy, is pushing the galaxies apart. This "stuff" has been dubbed dark energy, but to give it a name is not to understand it. Whether dark energy is some new kind of dynamical fluid, not known to physics, or whether it is a name for some property of the vacuum of empty space, or whether it is a name for some modification to general relativity is not yet known.

The dark energy question is the most perplexing question in cosmology, but it is not the only one. Even before the universal expansion was discovered, there were unexplained aspects of the observed Universe that required a very short period, immediately after the Big Bang, where the Universe experienced an incredible burst of expansion called "inflation." The key assumptions of the inflationary Universe, is that the Universe at the time of the Big Bang was filled with an unstable form of energy whose nature is not yet known. It may have been the same as the dark energy we see today, or it may have been something else entirely.

The inflationary model predicts that the primordial energy would have been "lumpy" - i.e., unevenly spread out in space - due to a kind of quantum noise that arose when the Universe was extremely small. This pattern would have been transferred to the matter of the Universe and would have shown up in the photons that suddenly began streaming away freely at the moment of recombination. As a result, we would expect to see, and do see, a lumpy pattern in the Universe's "baby picture," produced by WMAP. This picture places a strong set of constraints on possible cosmological models.

But it still doesn't answer the question of what powered inflation. The problem is that inflation was over well before recombination, and the opacity of the Universe before recombination has effectively pulled down a curtain to cover the events we are interested in. Is there any way to see through the curtain? Fortunately, the answer is yes. There is another way to observe the Universe that does not involve photons at all. The key is gravitational waves, the only known form of information that can reach us, undistorted, from the instant of the Big Bang itself.

Gravitational Waves Escape from the Earliest Moments of the Big BangD

Gravity is one of the four fundamental forces of nature. Einstein's theory of gravity, general relativity, predicts not only the details of orbits in the Solar System and the expansionary behavior of the Universe, both of which are observed, but is also predicts the existence of waves of gravity that would be generated by an accelerated massive body. The Big Bang represents the biggest acceleration of the biggest collection of mass the Universe has to offer so it is expected that, even though the Big Bang was long ago and far away, some of its gravitational-wave echoes could still be detected.

NASA is considering several missions to study gravitational waves:

  • LISA
    LISA (Laser Interferometry Space Antenna) will consist of a trio of spacecraft flying in an equilateral triangle formation and tracking each other with lasers. When a gravitational wave passes through the space inside the triangle, the distances the laser signals will have to travel will change, and the tiny advances or delays in the arrival times may be detected. To keep actual motion of the spacecraft from looking like the effects of gravitational waves, microthrusters in each spacecraft will undo the effects of buffeting by the elements in interplanetary space. LISA will detect normal binary starts and will investigate binary black holes, but it is also designed in such a way as to be able to detect possible sources of inflationary energy such as an electroweak phase transition.
    + For more on LISA

  • Inflation Probe
    The Inflation Probe will seek the imprint of gravitational waves on the relic cosmic microwave background by observing the polarization of the background photons. These gravitational waves, seen as they were at the moment of recombination, should reveal if and how the "inflation" field stretched and smoothed out Universe.
    + For more on the Inflation Probe

  • Big Bang Observer
    The Big Bang Observer is a gravitational wave detector in the LISA mold, but attuned to see the gravitational waves produced by inflation itself. Like electromagnetic waves, gravitational waves cover a broad spectrum. Understanding the expansion history of the Universe requires measuring the gravitational wave relics from this era in at least two widely spaced frequencies. The Inflation Probe will search for the effects of waves with periods of billions of years' while the Big Bang Observer will seek a direct detection of waves with periods of 0.1-10 seconds. The combination of the two is expected to allow the nature of the inflationary mechanism to finally be determined.
    + For more on the Big Bang Observer

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