Outline

1. Using the Expansion to Measure the Age of the Universe
2. The Fate of the Universe: Crunch or Coast?

Terms to Know

1. Using the Expansion to Measure the Age of the Universe

We can find out by "running the movie backwards" again to see how long the expansion has been taking place.

ROUGH ESTIMATE: If H = 70 km/s/Mpc, then a galaxy 100 Mpc away is receding from the Milky Way at 7000 km/s. We can use the standard distance = speed x time equation ( D = V x T, e.g., 6 km/h x 2 hours = 12 km) to find T, the age of the Universe:

T = distance / speed = 100 Mpc / 7000 km/s = 100 x 10⁶ pc/Mpc x (3 x 10¹³ km/pc) / 7000 km/s = 4.3 x 10¹⁷ seconds = 1.4 x 10¹⁰ years, about 14 billion years. This is called a Hubble Time, the age of the Universe determined directly from the rate of expansion.

Is the Hubble Time consistent with the ages of stars according to our understanding of stellar evolution? Just barely! The oldest globular clusters are evidently about 13 billion years old, and the age of the Universe seems to be about 14 billions years old -- but maybe just a little bit less!

Now you can see why it is so important to measure the value of the Hubble Constant!

2. The Fate of the Universe: Crunch or Coast?

Is there enough mass and therefore gravity to turn around the expansion, bringing all matter crashing back together in a gigantic "Big Crunch"? Or is there enough momentum from the expansion to overcome gravity and maintain the expansion forever?

This is one of the biggest questions in astronomy today, and many researchers have devoted their careers to finding the answer. It may be that within just the next few years we will know for sure, so stay tuned!

There are three basic possibilities:

1. Universe not dense and massive enough to halt expansion: open Universe
2. Universe has critical density: expansion slows, slows, slows, but never stops: flat Universe
3. density is high enough to halt expansion and turn it around to make a Big Crunch: closed Universe; could repeat indefinitely!

Today, the critical density is 9 x 10^-30 grams/cm³ -- about 6 H atoms per cubic meter, or 1 Milky Way galaxy per Mpc³. If we add up all the mass in stars, galaxies, dust, gas, and dark matter we can detect in the Universe, we get only about 30% of that -- so it looks at this point as if the Universe is open, and will expand forever.

In fact, there is some recent evidence that the expansion is actually speeding up, due to "extra energy" that exists even in apparently empty space.

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Astro 100