In the 1920s, American astronomer Edwin Hubble and others found that the Universe is expanding by observing that most galaxies are receding from the Milky Way, and that the farther they are from the Milky Way, the quicker they are receding. The Hubble constant was named from the generally constant relation between speed and distance. Hubble discovered that galaxies withdrew 500 kilometres per second quicker for every extra megaparsec (approximately 3.26 million light years) of distance, resulting in the Hubble constant of 500 kilometres per second per megaparsec.
As measurement techniques improved over the decades, astronomers reduced the estimate significantly lower. In the 1990s, Freedman pioneered the use of the Hubble Space Telescope to (appropriately) measure the Hubble constant, calculating a value of about 72 with a 10% error margin. The most accurate measurements have been conducted so far by a team led by Nobel laureate Adam Riess at Johns Hopkins University in Baltimore, Maryland, and the newest value is 74, with an error margin of just 1.91 percent 2.
However, a different attempt from the previous decade has put a wrench into the plans. Scientists with the European Space Agency’s Planck mission mapped the cosmic microwave background, or remnant radiation from the Big Bang, and used it to compute the Universe’s fundamental features. They computed the Hubble constant to be 67.8 based on typical theoretical assumptions about the cosmos.
The difference between 67.8 and 74 may appear insignificant, but as both procedures have improved, it has become statistically significant. As a result, scientists have begun to wonder if the cause of the mismatch resides in the conventional cosmological theory, known as the CDM, which asserts the presence of unseen dark matter particles as well as a mysterious repulsive force known as dark energy. They have, however, struggled to come up with a change to the theory that would fix the problem while remaining compatible with all we know about the Universe. “It’s difficult to look at CDM and understand where the loose threads are,” says Rocky Kolb, a cosmologist at the University of Chicago.
Freedman’s method improves on a critical component of the Hubble measurement method, yielding a result of 69.8.
The difficult element of determining the Hubble constant is determining the distances between galaxies with accuracy. Hubble’s first estimate was based on seeing specific brilliant stars known as Cepheids to determine the distances of neighbouring galaxies. In the early twentieth century, astronomer Henrietta Swan Leavitt discovered that the brightness of these stars could be predicted. She could calculate how far away the stars were by gauging how bright they appeared on photographic plates. Standard candles are the term used by astronomers to describe such markers.
However, researchers have been looking for better standard candles than Cepheids, which tend to exist in crowded, dust-filled locations, causing estimations of their brightness to be skewed. “The only way to get to the bottom of this is to have independent methods, and we’ve had no checks on the Cepheids up to this point,” says Freedman, who has devoted her career working to improve the precision and accuracy of Cepheid observations. Kolb says, “She knows where all the bodies are buried.”
Instead of Cepheids, Freedman and her colleagues utilized red giants ancient stars that have gotten puffed out and supernovae explosions as their standard candles, which act as signposts for more distant galaxies.
How is the Hubble rate determined?
, where H0 is the proportionality constantHubble constantbetween the “proper distance” D to a galaxy, which, unlike the comoving distance, can fluctuate over time, and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmological time coordinate. (For more on the nuances of this notion of “velocity,” see “Uses of the Proper Distance.”)
What is the recession velocity?
As the cosmos expands, the pace at which an extragalactic astronomical object recedes (becomes more distant) from an observer is called recessional velocity. The wavelength shifts of spectral lines emitted by the object, known as the object’s cosmic redshift, can be measured.
How quickly do galaxies rotate?
The Milky Way rotates at a speed of 130 miles per second (210 kilometers per second), but a recent study finds that dark matter has slowed the rotation of its bar by at least 24% since its birth roughly 14 billion years ago.
# 7: HUBBLE’S LAW
In other words, the recession velocity of a galaxy is proportional to its distance, as measured by its Doppler shift. Hubble’s constant H is the proportionality constant.
- Hubble’s law is a non-exact empirical relationship determined by observation. Within the Big Bang theory, it has a natural interpretation.
- It’s tough to calculate Hubble’s constant.
- The best current estimate is H 70 km/s/Mpc, but H might be as high as 90 or as low as 50 depending on observational uncertainties.
- A primary goal of observational cosmology is to improve the measurement of H.
(1) Given H=70 km/s/Mpc, what is the recession velocity of a galaxy 10 Mpc away?
(2) I determine that a galaxy is retreating from the earth at 7000 km/s by detecting its Doppler shift.
How far away is the galaxy if H=70 km/s/Mpc?
What is the formula for calculating expansion velocity?
One of the most important formulas of the twentieth century is Hubble’s Law. It depicts the expansion of the Universe by depicting the movement of distant galaxies away from us.
1. The distance between us and NGC 123 is 1,320 km/s, while the Hubble Constant is 70 km/s/Mpc.
According to Hubble’s Law, how far away is the galaxy?
2. NGC 2342 is a galaxy with a velocity of 5,690 km/s and a distance of 74 Mpc.
Based on these numbers, what is the Hubble constant’s value?
3. The galaxy NGC 4442 is 120 million light-years away.
How fast should NGC 4442 be traveling owing to the expansion of the Universe if the Hubble Constant is 68 km/s/Mpc?