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The universe's dark mysteries

The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, a member of AAAS, Brian Schmidt, and Adam Reiss for their discovery that the rate of expansion of the universe is accelerating. This is the latest in a series of surprising changes in our view of the universe driven by advanced astronomical measurements and open minds.

Hubble's 1929 spectro-graphic study of Cepheid variable stars revealed that the redshift of a galaxy was proportional to its distance, meaning that the universe was expanding uniformly. Such expanding universes could be modeled using general relativity.

Later, discovery of the uncanny homogeneity of the cosmic background radiation led Alan Guth to propose in 1980 an inflationary phase in the early history of the Universe, wherein the universal length scale suddenly increased by a factor of ~1026. This produces a flat homogeneous expanding Universe whose CBR agrees with microwave observations.

In their Nobel Prize work, Perlmutter, Schmidt, and Reiss headed two teams in using observations of Type-Ia supernovae to determine the distance to galaxies having known redshifts. They found that supernovae in distant galaxies were about 25 percent less luminous than expected, meaning that the expansion of the universe has been accelerating for the past nine billion years. This discovery shook the foundations of modern cosmology.

Adjusting cosmology to attribute for an accelerating expansion has lead to today's current standard model of Big Bang cosmology. Today the standard is the L-CDM model, a cold dark matter model with dark energy making up the majority of our universe and counteracting the force of gravity to drive the acceleration.

  • Dark matter? Dark matter supplies additional gravitational interactions needed to reconcile the dynamics of large matter-based structures with observations.
  • Dark energy? Dark energy is usually modeled using Einstein's cosmological constant, which adds a uniform positive energy density having negative pressure to a model of the Universe. The negative pressure drives the accelerating expansion.

Remarkably, in these models only about five percent of the mass-energy of the universe is in the form of normal matter, while 23 percent is dark matter and the remainder is dark energy. Neither dark matter nor dark energy have been directly detected, and their nature and properties remain largely unknown.

Will the universe end in fire, or in ice? We still do not know — either possibility lies within the uncertainties of the observational data. The cosmological community will continue to seek new forms of measurement in the quest to understand where the universe came from, and how it will evolve into the future. 

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