January 28, 2017

The following information was taken from and NASA.

Until just recently I did not know there was a Hubble Constant.  The term had never popped up on my radar.  For this reason, I thought it might be noteworthy to discuss the meaning and the implications.


The Hubble Constant is the unit of measurement used to describe the expansion of the universe. The Hubble Constant (Ho) is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. This long-sought number indicates the rate at which the universe is expanding, from the primordial “Big Bang.”

The Hubble Constant can be used to determine the intrinsic brightness and masses of stars in nearby galaxies, examine those same properties in more distant galaxies and galaxy clusters, deduce the amount of dark matter present in the universe, obtain the scale size of faraway galaxy clusters, and serve as a test for theoretical cosmological models. The Hubble Constant can be stated as a simple mathematical expression, Ho = v/d, where v is the galaxy’s radial outward velocity (in other words, motion along our line-of-sight), d is the galaxy’s distance from earth, and Ho is the current value of the Hubble Constant.  However, obtaining a true value for Ho is very complicated. Astronomers need two measurements. First, spectroscopic observations reveal the galaxy’s redshift, indicating its radial velocity. The second measurement, the most difficult value to determine, is the galaxy’s precise distance from earth. Reliable “distance indicators,” such as variable stars and supernovae, must be found in galaxies. The value of Ho itself must be cautiously derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small.

The units of the Hubble Constant are “kilometers per second per megaparsec.” In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. (A megaparsec is 3.26 million light-years.) For example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s.

The cosmos has been getting bigger since the Big Bang kick-started the growth about 13.82 billion years ago.  The universe, in fact, is getting faster in its acceleration as it gets bigger.  As of March 2013, NASA estimates the rate of expansion is about 70.4 kilometers per second per megaparsec. A megaparsec is a million parsecs, or about 3.3 million light-years, so this is almost unimaginably fast. Using data solely from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), the rate is slightly faster, at about 71 km/s per megaparsec.

The constant was first proposed by Edwin Hubble (whose name is also used for the Hubble Space Telescope). Hubble was an American astronomer who studied galaxies, particularly those that are far away from us. In 1929 — based on a realization from astronomer Harlow Shapley that galaxies appear to be moving away from the Milky Way — Hubble found that the farther these galaxies are from Earth, the faster they appear to be moving, according to NASA.

While scientists then understood the phenomenon to be galaxies moving away from each other, today astronomers know that what is actually being observed is the expansion of the universe. No matter where you are located in the cosmos, you would see the same phenomenon happening at the same speed.

Hubble’s initial calculations have been refined over the years, as more and more sensitive telescopes have been used to make the measurements. These include the Hubble Space Telescope (which examined a kind of variable star called Cepheid variables) and WMAP, which extrapolated based on measurements of the cosmic microwave background — a constant background temperature in the universe that is sometimes called the “afterglow” of the Big Bang.


The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment.

According to the standard theory, our universe sprang into existence as “singularity” around 13.7 billion years ago. What is a “singularity” and where does it come from? Well, to be honest, that answer is unknown.  Astronomers simply don’t know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of “black holes.” Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called “singularities.” Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something – a singularity. Where did it come from? We don’t know. Why did it appear? We don’t know.

After its initial appearance, it apparently inflated (the “Big Bang”), expanded and cooled, going from very, very small and very, very hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we are inside of it: incredible creatures living on a unique planet, circling a beautiful star clustered together with several hundred billion other stars in a galaxy soaring through the cosmos, all of which is inside of an expanding universe that began as an infinitesimal singularity which appeared out of nowhere for reasons unknown. This is the Big Bang theory.


The illustration below shows the three steps astronomers used to measure the universe’s expansion rate to an unprecedented accuracy, reducing the total uncertainty to 2.4 percent.

Astronomers made the measurements by streamlining and strengthening the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near and far from Earth.

Beginning at left, astronomers use Hubble to measure the distances to a class of pulsating stars called Cepheid Variables, employing a basic tool of geometry called parallax. This is the same technique that surveyors use to measure distances on Earth. Once astronomers calibrate the Cepheids’ true brightness, they can use them as cosmic yardsticks to measure distances to galaxies much farther away than they can with the parallax technique. The rate at which Cepheids pulsate provides an additional fine-tuning to the true brightness, with slower pulses for brighter Cepheids. The astronomers compare the calibrated true brightness values with the stars’ apparent brightness, as seen from Earth, to determine accurate distances.

Once the Cepheids are calibrated, astronomers move beyond our Milky Way to nearby galaxies [shown at center]. They look for galaxies that contain Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same amount of brightness. The astronomers use the Cepheids to measure the true brightness of the supernovae in each host galaxy. From these measurements, the astronomers determine the galaxies’ distances.

They then look for supernovae in galaxies located even farther away from Earth. Unlike Cepheids, Type Ia supernovae are brilliant enough to be seen from relatively longer distances. The astronomers compare the true and apparent brightness of distant supernovae to measure out to the distance where the expansion of the universe can be seen [shown at right]. They compare those distance measurements with how the light from the supernovae is stretched to longer wavelengths by the expansion of space. They use these two values to calculate how fast the universe expands with time, called the Hubble constant.


Now, that’s simple, isn’t it?  OK, not really.   It’s actually somewhat painstaking and as you can see extremely detailed.  To our credit, the constant can be measured.


This is a rather, off the wall, post but one I certainly hope you can enjoy.  Technology is a marvelous thing working to clarify and define where we come from and how we got there.


2 Responses to “HUBBLE CONSTANT”

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