How old is the universe?

The best available information indicates that the age of the universe is 13.7 billion years. Hubble has helped to measure the age of the universe using two different methods. The first method involves measuring the speeds and distances of galaxies. Because all of the galaxies in the universe are generally moving apart, we infer that they must all have been much closer together sometime in the past. Knowing the current speeds and distances to galaxies, coupled with the rate at which the universe is accelerating, allows us to calculate how long it took for them to reach their current locations. The answer is about 14 billion years. The second method involves measuring the ages of the oldest star clusters. Globular star clusters orbiting our Milky Way are the oldest objects we have found and a detailed analysis of the stars they contain tells us that they formed about 13 billion years ago. The good agreement between these two very different methods is an encouraging sign that we are honing in on the universe’s true age.

How big is the universe?

We can observe only a portion of the entire universe. Because the universe is only about 14 billion years old, light has only had about 14 billion years to travel through it. Therefore, the most distant regions of the universe we can see are about 14 billion light-years away. This is the extent of the "observable universe," but the entire universe is probably much larger. It could even extend infinitely in all directions.

In what sense is the universe expanding?

We live in an expanding universe. All of the galaxies (vast collections of stars similar to, but outside of, our own Milky Way galaxy) that populate the universe – including our Milky Way – are moving away from each other. How quickly galaxies move away from one another depends on their relative distance. From our viewpoint, the farther away another galaxy is, the faster it moves away from us. This is called the Hubble Law (after the American astronomer Edwin Hubble, who discovered the cosmic expansion in the late 1920s from the 100-inch Hooker Telescope on Mount Wilson near Pasadena, CA).

Although we see galaxies moving away from us in all directions, this does not mean that our galaxy is in the center of some sort of explosion; observers in other galaxies would see the same thing. It only means that the space between all galaxies is growing larger.

Here is a simple example to help you understand the expansion of the universe. Take an uninflated balloon and cover it with dots. Each point represents a galaxy. When you inflate the balloon, the points move away from one another. Notice that no matter which point you choose to represent your location, all points move away from it.

Even though the balloon is stretching uniformly, dots separated by a greater distance move away from each other at a faster speed; the velocity is proportional to the distance. This is the Hubble Law. It happens everywhere on the balloon – and in our universe.

How do we know that galaxies are moving away from ours? It isn't because we see them getting smaller–they don't move that fast. But we observe the frequency of the light from these galaxies (and hence their color) is being shifted to the red end of the spectrum, much as the pitch of a siren atop an ambulance shifts lower as the ambulance moves away from you. The amount of this shift can be measured, and this number is called the "redshift."

The Hubble Space Telescope has contributed to measurements of the expansion of the universe.

Find out more about the expansion of the universe from HubbleSite:

What are some of the key findings learned from the Hubble Deep Field image?

In the document titled: Summary of Key Findings from the Hubble Deep Field you will find information under these headings:

  • Small Galaxies in the Early Universe
  • Open versus Closed Universe
  • Disturbed Galaxies
  • Stellar Baby Boom
  • In Search of Hidden Stars
  • Missing Mass – Still Missing

This document can be found at: http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/41/astrofile/#2

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