The age of the Universe
What is the age of the universe? Take Hubble's law. If the universe
expanded at the same rate, then one can quickly find the age of the universe.
This is done following easy steps. The Hubble's constant can be measured.
The original value, H = 500 Km/sec/Mpc, has been proven incorrect, thanks
to better determination of distances to galaxies. The current value is
between 50 and 100 Km/sec/Mpc.
Using 1 Mpc = 106 pc = 3.26 106 light years
= 3 1019 Km.
That line is characterized by the expression: v = H d, where v is the measured velocity of the galaxy, d is its distance (measured in an independent way) and H is the slope of the line. There is some scatter in the data points, so there is some uncertainty on how to draw the line and on the value of the slope. Notice, however, that if one is able to measure galaxies further and further away, the uncertainty in how to draw the slope diminishes. This is the reason why astronomers are trying to measure distances to very far away galaxies.
to measure the distance is to look at the light emitted by supernovae.
In supernova type I, a white dwarf (a star that is slowly dying out because
of the lack of fuel) accretes
material from a companion star. The inflow of material creates an instability
and eventually an explosion whose characteristics (such as brightness)
can be calculated. By measuring how bright or dim the supernova looks to
us on Earth, an estimate of the distance can be obtained. Using supernovae
as standard candles, it was found that the Hubble constant has values in
the range of: 60-70 Km/sec/Mpc. Unfortunately, it is found that some astronomical
objects, such as globular clusters and white dwarfs, are as old or even
older than this estimate of the age of the universe. The determination
of the age in this latter case comes from radioactive decay dating of elements.
The rate of the expansion and the cosmological constant. "My own reaction is somewhere between amazement and horror", says Brian Schmidt, team leader of a search of supernovae. In a recent announcement, he brought evidence that the universe is expanding at a higher rate that it has been known before. They measured supernovae type I and found them further away than expected; they analysis showed that the rate of the expansion of the universe through ages. In a geometrically flat universe, consistent with the theory of inflation, the expansion should gradually slow so as the universe would reach infinity at an infinite time. There is no such evidence of a slowdown, the team says, on the contrary, the rate is increasing and the expansion is speeding up. There is no accepted mechanism that would do that.
theory, postulating antigravity or the cosmological constant, goes back
to the work of Einstein. Although general relativity predicts a dynamic
and expanding universe, Einstein resisted that idea and introduced, in
his own equations, a term, called the cosmological constant, that counterbalances
gravity. This cosmological constant, whose effects are significant only
at very large distances, would pull matter apart instead of pushing it
together as gravity does (when its value is > 0). The effect of a positive
value of the cosmological constant is to increase the age of the universe
beyond the one obtained from an analysis of the Hubble's constant. No such
evidence has been found nor mechanism explaining this new force. The recent
announcement opens the door to the introduction of the cosmological constant;
the team suggests that the cosmological pulling force could account for
an accelerated expansion of the universe.