Find out how researchers at Princeton are pinning down the Hubble Constant, which tells us how fast our universe is expanding by using gravitation waves.
Every society wonders about the big questions of the universe. How did it begin? Where did we come from? How will it all end? They observe what they can and tell each other stories to explain their world and give life meaning.
A hundred years ago, scientists thought they had a pretty good idea of how the universe worked. They believed that it had always been here and that it always would be here. Our planet orbited around the sun, which they knew was a kind of star. There were lots of other kinds of stars that were clustered together into the galaxy. Then, outside the galaxy was empty space. Stars would come and go, but as for the big picture, the universe was in a steady state that would last forever.
Then, in 1925, Edwin Hubble pulled the rug out from under this comfortable cosmology. Using the brand-new 100-inch Hooker Telescope at Mount Wilson, Hubble started studying the nebulae in the night sky. He noticed that there were Cepheid variable stars in these nebulae. Cepheid variables are giant stars that vary in brightness over time.
Hubble pulled the rug out from under cosmology
The unsung hero of the story is Henrietta Leavett. She graduated from Radcliffe College and worked at the Harvard Observatory as a “computer”. Women weren’t permitted to actually use the telescope at the observatory in those days. Instead, she had the job of measuring and cataloging the brightness of stars on photographic plates. In the course of her work, she discovered the relationship between the luminosity and the period of Cepheid variable stars. She has never been properly recognized for her crucial discovery.
Using Leavett’s formula, Hubble was able to work out the distance to the Cepheids that he was observing. The result was astounding. They were 860,000 light years away. That meant that they couldn’t possibly be part of our galaxy. Concentrating on the Andromeda nebula, Hubble realized that Andromeda was another galaxy like our own. Andromeda was close to the same size and it held about the same about of matter as the Milky Way galaxy.
As Hubble continued his work, he found something even more stunning. In 1929, Hubble measured the Doppler Effect, or red shift, of all the galaxies he had discovered. He proved that they were all moving away from one another. The universe was not in a steady state. Everything was expanding at tremendous speed. What’s more, the further away a galaxy was, the faster it traveled. If a galaxy is twice as far away, it goes twice as fast. It it’s ten times as far away, it goes ten times as fast. This is called Hubble’s Law.
Hubble Constant: rate of universe expansion
Part of Hubble’s law is an element called the Hubble Constant. This is the base rate at which the universe is expanding. The exact value of the Hubble Constant has been hard to pin down. Today’s scientists, using the Hubble Space Telescope (named after Edwin Hubble) have narrowed it down to approximately 72 kilometers per second per megaparsec.
That’s where this week’s news comes in. Researchers at Princeton University released some new findings on the Hubble Constant in the journal Nature Astronomy. Up until now, there have been two ways to measure the Hubble Constant.
To deal with all this, Professor Kenta Hotokezaka and Kento Masuda from Princeton, Ore Gottlieb and Ehud Nakar from Tel Aviv University in Israel, Samaya Nissanke from the University of Amsterdam, Gregg Hallinan and Kunal Mooley from the California Institute of Technology, and Adam Deller from Swinburne University of Technology in Australia found a new way to measure the Hubble Constant using mergers of neutron stars.
two existing techniques give different answer
Scientists can study the cosmic microwave background from the Big Bang, or they can use a kind of exploding star called a Type 1A supernova. The trouble is, the two techniques don’t give the same answer. The supernova technique yields a faster rate than the microwave background technique. Anyway, neither technique is a direct measurement.
Neutron stars carry high levels of energy. When two of them merge, they release powerful gravitation waves and massive volumes of material. The wave burst is called a standard siren and is easy to detect and distinguish. To refine their observations, they also joined images from radio telescopes into a movie of the merger’s fireball and its aftermath.
n the end, the research team found that the Hubble Constant is between 65.3 and 75.6 kilometers per second per megaparsec. As you can see, this result is right in line with results using other techniques. The trouble is, it still isn’t accurate enough to resolve the discrepancy. Yet, in time, this technique will yield the result cosmologists need.
pinning down exact value of Hubble Constant
Repeating this experiment 15 more times would deliver the data to pin down the exact value of the Hubble Constant. Another way would be to measure the gravitation waves alone from 50 to 100 similar mergers. One way or another, we will arrive at a precise number for the Hubble Constant.
Every culture has a cosmology. They may believe in Father Sky and Mother Earth or that our world is a giant turtle. Our culture takes pride in having a more scientific cosmological model. Even so, we need our story as much as indigenous cultures need theirs.
The Hubble Constant is the key to figuring out the origin and the fate of our universe. It may be the most important number in science. Every story needs a beginning, a middle and an end. We won’t have filled in the details of our story until we can pin down the precise value of the Hubble Constant. This new technique will take us closer to that goal.
We always have more to learn if we dare to know.
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