Hubble Space Telescope and the Accelerating Universe

Orbiting high above the turbulence of the earth’s atmosphere, the Hubble Space Telescope (HST) is providing breathtaking views of astronomical objects never before seen in such detail.  The steady diffraction-limited images allow this medium-size telescope to reach faint galaxies of 30th stellar magnitude.  Some of these galaxies are seen as early as 2 billion years after the Big Bang in a 13 billion year old universe.  Up until recently, cosmologists assumed that all of the laws of physics and astronomy applied back then as they do today.  Now, using the discovery that certain supernovae are “standard candles”, astronomers have found that the universe is expanding faster today than it was back then: the universe is accelerating in its expansion.

Einstein published the general theory of relativity in 1916. It describes the nature of gravity in large-scale systems such as planets, stars and nebulae.  In the decades around 1910, the universe was thought to be static.  As part of the development of general relativity, Einstein invented the “cosmological constant”, a negative energy to balance gravity (a form of positive energy) and keep the universe from collapsing.  In 1927, Edwin Hubble and Vesto Slipher discovered that the spiral nebulae were receding from each other with velocities proportional to their mutual distances.  Einstein thought he no longer needed the cosmological constant and called it “… the greatest blunder of my life”.  The universe was expanding so fast it might never slow down.

In recent years, a major breakthrough has been made in the field of cosmology.  Using HST and ground-based images, astronomers and physicists have discovered a new means for measuring the distances to faint galaxies.  Using the light from exploding stars called supernovae of type Ia, observers can measure distances by comparing their known intrinsic brightness to their apparent brightness’s.  At maximum light, these supernovae are as bright as the sum of all the stars in their parent galaxies combined.  This means that Ia supernovae can be seen very far away – out in space and back in time to when the first galaxies were formed. This has led to the hypothesis that the universe is accelerating.

In comparison with distances derived from Doppler shifts in their spectra, supernovae distances in faint young galaxies show that the universe was expanding more slowly (in fact decelerating) than it does today.  The simplest hypothesis is that because of the Big Bang, the mean density of the universe is decreasing rapidly with time while the cosmological constant (also known as dark energy) is there, unchanging, throughout space.  Today, dark energy has command of the universe.

It appears that Einstein was right after all.

H. John Wood
NASA/GSFC, Code 551
Greenbelt, MD 20771