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Q. What is the Hubble Constant? — Mary Elizabeth A.
A. Hold onto your hat! This gets a bit technical.
First, let's go back to 1929 when the American astronomer Edwin P. Hubble was studying light arriving at Earth from distant galaxies.

Red Shift. He noticed that the colors of light from the stars in the galaxies didn't seem to be exactly what they should be. Instead, they appeared to be shifted toward the red end of the spectrum of colors.

He decided that such red shifts must be because those other galaxies were moving away from our own Milky Way galaxy.

Definition. The precise definition of red shift is the increase in the wavelength divided by the original wavelength. For a given relative velocity, this quantity is the same for all wavelengths or colors.

Thus, the speed at which a galaxy is moving away from us is measured by its red shift.

Hubble's Law. Hubble was the first to realize that galaxies are racing away from each other at a proportional distance. The farther away, the faster they recede in a linear relationship.

Hubble's law says that the distances between galaxies are continuously increasing and therefore the Universe is expanding.

Hubble Constant. Within Hubble's Law, Hubble's Constant depicts the relative rate at which the scale of the Universe changes over time.

The Hubble Constant is one of the most important numbers in cosmology. It is used to estimate the size and age of the Universe. For instance, the reciprocal of the Hubble Constant is thought of as a time that represents the age of the Universe.

A low Hubble Constant suggests the Universe is expanding slowly. Therefore, it must be very old to have reached its current size.

On the other hand, a high Hubble Constant suggests a rapid expansion and, therefore, a relatively young Universe

What's the Number? The number is of great importance to astronomers because it indicates the rate at which the Universe has been expanding since the primordial “Big Bang.”

The units of the Hubble constant are “kilometers per second, per Megaparsec.” A Megaparsec (Mpc) is 3.26 million light-years.

For each Megaparsec of distance, the velocity of a distant object appears to increase by some value.

Unfortunately, Hubble underestimated the distances to the galaxies, which makes his calculation of the expansion rate five or ten times too large.

Astronomers in the 21st century believe the number is somewhere between 46 and 57 miles per second per megaparsec (57 and 73 kilometers per second per megaparsec). As you can see, there is still is a significant amount of uncertainty in the value of the Hubble Constant.

So how old is everything? In recent years, two teams at the Carnegie Observatories measured the Hubble Constant by different methods and came up with differing answers. Their current estimates of the age of the Universe are either 9-12 billion years or 11-15 billion years.

Doppler Effect. The change in the color (wavelength) of light that results from the relative motion of a distant star and the receiver of the light is an example of the Doppler effect.

Calculating the Constant. Scientists refer to the Hubble Constant in this way:

• H₀ is the current value of the Hubble Constant
  
  
• v is the galaxy's radial outward velocity (motion along our line-of-sight)
  
  
• d is the galaxy's distance from Earth
  
  
• H₀ = v/d is the Hubble Constant stated as a simple mathematical expression

Hubble Space Telescope. Helping scientists calculate a definitive value for the Hubble Constant was one of the major goals for the new Hubble Space Telescope when it was launched in 1990.

Hubble's incorrect computation had the constant at about 500 km/s/Mpc.

Until the launch of the Hubble Telescope, the range of measured values for the expansion of the Universe was from 50 to 100 km/s/Mpc.

Now the precise value of the Hubble Constant is said to be 70 km/s/Mpc, with an uncertainty of ten percent.

This means that a given galaxy appears to be moving 160,000 mph faster for every 3.3 million light-years away from Earth.

12 Billion Years Old. Before the advent of the Hubble Space Telescope, astronomers couldn't decide if the Universe is 10 billion or 20 billion years old.

The size of the Universe is so vast that astronomers were unable to find definitive answers to their basic questions about the origin and the eventual fate of the Universe.

They came closer to answers in 1999 when a NASA Hubble Space Telescope Key Project team of Carnegie Observatories -- 27 astronomers at 13 different U.S. and international institutions -- revealed its calculation the age of the Universe with more-precise accuracy.

After their eight-year effort to measure the far-flung galaxies of the expanding Universe, the Key Project astronomers decided the Universe is approximately 12 billion years old.

Cepheid Variables. The team used the Hubble Telescope to look at 18 galaxies ranging as far away as 65 million lightyears.   [What is a lightyear?]

During their search of the vast reaches of the Universe, they uncovered almost 800 Cepheid variable stars -- a special class of pulsating stars used for accurate distant measurement.

The rare Cepheids provide what astronomers call a “standard candle” for calculating intergalactic distances. The team used the Cepheid stars to calibrate their distance measurements.

Combining the Hubble Constant measurement with estimates for the density of the Universe, the team computed that the Universe is approximately 12 billion years old—similar to the oldest stars.

Younger Than Its Oldest Stars? The team's results cleared up the mystery of the Universe seeming to be younger than its oldest stars.

Age is computed using the expansion rate from precise distance measurements. The computed age is refined on whether the Universe appears to be accelerating or decelerating, given the amount of matter seen in space. A rapid expansion rate makes it seem the Universe has not required as much time to reach its present size, so it is younger than if it were expanding more slowly.

The team said the age estimate holds true if the Universe is below what they called a “critical density,” -- a point at which it is balanced between expanding forever or collapsing.

On the other hand, it could be that some mysterious force is pushing the galaxies farther apart. If that is the case, the Hubble Constant measurement would suggest an even older Universe.

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