Hubble constant is a measure of the rate at which the universe is expanding. It serves as a key part of Hubble’s law, one of the most important observations in cosmology (the study of the universe’s structure and development). Hubble’s law describes the way distant galaxies recede (move farther away) as space expands. The Hubble constant and Hubble’s law are named for their discoverer, the American astronomer Edwin P. Hubble. Scientists can use the Hubble constant to estimate the universe’s current age.
Evidence indicates that the universe has not always expanded at the same rate, so the value of the Hubble constant must change over time. When scientists talk about the Hubble constant, they often are referring to the constant’s current value.
The expansion of space.
Nearly all scientists agree that space began expanding with a cosmic explosion called the big bang. That expansion continues today.
To understand the effects of this expansion, imagine a room that doubled in size over the course of a year. Two objects in the room that began 5 feet (1.5 meters) from each other would end up 10 feet (3 meters) apart. Their rate of recession would equal the increase in the distance between them (10 feet – 5 feet = 5 feet) divided by the time (1 year), or 5 feet per year. Similarly, objects that started 50 feet (15 meters) apart would finish 100 feet (30 meters) apart, having receded at a rate of 50 feet per year. As you can see, the farther apart two objects began, the more rapidly they would recede from each other.
Hubble’s law.
In 1929, Hubble discovered a similar relationship while measuring the movement of faraway galaxies. He found that the farther such a galaxy was from our own Milky Way, the more rapidly it appeared to be receding. Hubble concluded that distant galaxies recede at a rate proportional (directly related) to their distance. This idea became known as Hubble’s law.
According to Hubble’s law, two galaxies that are 20 million light-years apart are receding from each other twice as rapidly as are two galaxies that are 10 million light-years apart. One light-year is the distance that light travels in a vacuum in a year, about 5.88 trillion miles (9.46 trillion kilometers).
Hubble’s law does not apply to galaxies that are relatively close together or objects, such as stars and planets, that are within the same galaxy. At such short distances, the gravitational attraction between galaxies and other objects prevents them from receding as described by Hubble’s law.
The Hubble constant
is the ratio of two galaxies’ rate of recession from each other to the distance between them. Using the constant, Hubble’s law can be written v = H X d. In this equation, v stands for the rate of recession, often expressed in kilometers per second. The symbol d represents the distance between the galaxies, usually measured in megaparsecs. One megaparsec equals 1 million parsecs or about 3.26 million light-years. The symbol H represents the Hubble constant, expressed in kilometers per second per megaparsec.
To apply the equation, consider two galaxies: (1) our own Milky Way and (2) a galaxy that is 3 megaparsecs from the Milky Way. Astronomers have measured the current value of the Hubble constant to be about 70 kilometers per second per megaparsec. According to the equation, the two galaxies are receding from each other at a speed of 70 kilometers per second per megaparsec X 3 megaparsecs, or 210 kilometers per second.
The age of the universe
can be estimated using the Hubble constant. If we know the distance between two galaxies and how fast they are receding from each other, we can determine approximately how long ago they were right next to each other, the situation around the time of the big bang. This calculation uses one of the most basic equations in physics, written t = d/v. In this equation, t stands for time, d stands for distance and v stands for velocity. If we let d equal the distance between the galaxies and v equal their rate of recession, then t will equal the approximate time since the big bang—that is, the age of the universe.
Remember that Hubble’s law tells us that v = H X d. Entering this value into the equation t = d/v, we find that t = d/(H X d), or simply t = 1/H. So, the approximate age of the universe equals 1 divided by the Hubble constant. For a Hubble constant of 70 kilometers per second per megaparsec, this calculation works out to roughly 14 billion years, a figure that closely agrees with other estimates of the universe’s age.
The equation t = d/v gives an approximate result for the age of the universe because space has not always expanded at the same rate. The gravitational attraction among galaxies tends to work against the expansion of space. Astronomers have also found evidence that the rate of expansion is increasing, perhaps due to a little-understood form of energy called dark energy.
Measuring the Hubble constant
involves determining the rate at which distant galaxies recede from us and measuring the distance to those galaxies. Astronomers determine the rate of a galaxy’s recession by measuring its redshift. Redshift is a stretching of the wavelengths of certain kinds of radiation sent out by the galaxy. See Redshift .
Measuring the distance to a galaxy can prove more complex. A simple way to find this distance is to use a galaxy’s redshift to determine its rate of recession and then to enter this value into the equation for Hubble’s law. This method requires using a known estimation for the Hubble constant, however, so it cannot be used to determine the constant’s value.
In the late 1990’s, astronomers studying data from the Hubble Space Telescope used several methods for measuring distance to arrive at the estimated value of 70 kilometers per second per megaparsec for the Hubble constant. In 2003, scientists used data gathered by a satellite called the Wilkinson Microwave Anisotropy Probe to confirm the estimated value with even greater certainty.
See also Big bang ; Cosmology ; Gravitation (Expansion of the universe) ; Hubble, Edwin Powell .
