The Expanding Universe:

Accelerating Expansion

What happens in the future?

The expansion of the Universe is a key feature of the Big Bang cosmology and is a generic property of the Universe we inhabit.

The future fate of our expanding Universe should depend on how much matter (= mass = gravity) is in the Universe.

There are several possibilities:

(1) Recollapsing Scenario

    • Too much mass = gravity wins

    • Expansion reverses, and the Universe collapses to a Big Crunch.

(2) Critical Scenario

    • “Just right” = gravity and expansion in balance

    • Expansion slows and approaches a rate of zero (but never quite stops).

(3) Coasting Scenario

    • Too little mass = gravity loses

    • Expansion continues forever at a fixed rate.

We can’t tell the scenarios apart at the current time because all of them predict an observation of a (currently) expanding Universe.

We can’t look into the future to see what happens next, so we have to look for clues in the past.

Fortunately, this is easy! Because of the light speed travel time, everything we see around us in the Universe is on a time delay. More distant objects appear to us as they looked longer into the past.

Possibilities for the future of our expanding Universe. From left to right, they are (1) the recollapsing scenario, (2) the critical scenario, and (3) the coasting scenario. Each scenario predicts a successively older Universe, because the Big Bang would have had to happen further back in time to explain the particular expanding Universe that we find ourselves in right now (i.e., the observable Universe with a value of the Hubble constant around 70 km/sec per Mpc).

Credit: NASA & ESA [link]

The 1998 Surprise

Observations culminating in 1998 showed that Type Ia supernovae in very distant galaxies (looking at much earlier times in the Universe’s history) were fainter than expected based on their distances calculated from the Hubble Law (redshift).

This means:

    • The Universe is expanding faster now than in the past.

    • The rate of expansion of the Universe is accelerating!

    • This acceleration has been happening for the past 5 billion years.

Diagram showing the best match for expanding Universe models with Type Ia supernova observations, which provide a way to determine the size of the Universe at various times in the past. We cannot distinguish between the different Universe expansion scenarios right now (vertical dotted line) because all of them predict that we should currently see an expanding Universe. We cannot see into the distant future to distinguish between the scenarios, so we have to look at what happened in the past. The supernova observations strongly disagreed with the recollapsing scenario. Surprisingly, they also disagreed with the other two possible scenarios (critical and coasting)! Only a new and completely unexpected scenario in which the expansion of the Universe has been accelerating for the past 5 billion years matches the supernova observations.

Credit: © 2006 Pearson Education, Inc., publishing as Addison Wesley [link]

In the new scenario for the expanding Universe, the rate of expansion is increasing. This requires the Universe to be even older than the other three (rejected) scenarios.

Credit: NASA & ESA [link]

The astronomers whose work led to the discovery of the accelerating expansion of the Universe were awarded the Nobel Prize for Physics in 2011.

Gravity alone can explain the other three scenarios for the expanding Universe.

But what causes the acceleration of the expansion?

The answer to that question is coming in the next section...


5 Gyr of acceleration

  • Frieman, J. A., Turner, M. S., & Huterer, D. 2008, Annual Review of Astronomy and Astrophysics, 46, 385 – “Dark energy and the accelerating universe” (2008ARA&A..46..385F)