The expanding universe: discovery, controversies, and hope

We’ve known that there is a universe outside the Milky Way, and that it is expanding, for less than a century.

“Throughout the entire history of the universe, of knowing it’s expanding, there have been a tremendous number of controversies over it, and there’s still one that persists today,” said astrophysicist and author Ethan Siegel. Siegel, author of Beyond the Galaxy: How Humanity Looked Beyond Our Milky Way and Discovered the Entire Universe (World Scientific Publishing, 2015), spoke at last week’s meeting of the Rose City Astronomers in Portland, Oregon.

The controversy actually goes back to before the expansion was observed, to Albert Einstein. His equations describing general relativity suggested that gravity would collapse the universe onto itself, and as he believed the universe was static, he threw in a “cosmological constant” to push back against gravity. Einstein later called that his biggest blunder, though some wanted to let him off the hook for it when dark energy was proposed to do the exact same thing.

“I am here to tell you that this was Einstein’s reasoning and throwing this in there when he did was a super big blunder because the universe isn’t static,” Siegel said. Einstein should have trusted his theory, he said, and taken it to the next step.

The universe is expanding

By the 1920s Edwin Hubble observed a Cepheid variable star in the Andromeda “nebula” that indicated that it was far outside the Milky Way and a galaxy in its own right. Astronomers were also studying redshift as an indication for the speeds at which galaxies were receding from us. Siegel explained that through this, Hubble determined that the universe was expanding at a rate of 600km/sec/Mpc (kilometers per second per megaparsec.) This became the Hubble constant. But it wasn’t so constant.

Siegel noted that, knowing the size and expansion rate of the universe, you can figure its age by running the numbers in reverse and going back to the beginning, to the Big Bang. The resulting calculation determined that the universe was about two billion years old. Geologists at the time had already pegged the age of the Earth as at least four billion years.

“This was a problem for Hubble, because the universe isn’t allowed to be half the age of the Earth,” Siegel noted. “Either this expansion rate is wrong and this age for the universe is wrong, or the age of the Earth is wrong.”

It turns out that Hubble’s main mistake was in figuring that all variable stars are alike. Siegel said Walter Baade came along in the 1940s and discovered that they are not. Finding that most of the Cepheids Hubble had looked at were non-classical, they re-ran the numbers from Hubble’s data.

“As you accumulate more knowledge, as you accumulate a better understanding of what you’re actually loooking at, you can go back and get more useful science out of this data,” Siegel said. This second look doubled the distance to these stars and reduced the value for the Hubble constant to 270km/sec/Mpc. This in turn put the age of the universe at five billion years.

“That’s better,” Siegel noted. “The universe is older than Earth. That’s one problem solved.”

Narrowing it down

Siegel

Dr. Ethan Siegel, creator of the “Starts With a Bang” blog, gave a talk about the age and size of the universe to the Rose City Astronomers February 20. Photo: Greg Scheiderer.

As time went on astronomers developed the “distance ladder” for determining the vast distances in the universe. You first measured the distance to Cepheid variables within the Milky Way, then gauged the distances to other galaxies using Cepheids spotted there. Type 1a supernovae could be spotted really far out. As we learned more about the stars we got a little better at figuring distances.

Things got really interesting in the 1960s, according to Siegel. We discovered that we could determine the ages of stars by measuring their color and brightness. The Hertzsprung–Russell diagram told us that the oldest stars were between 14 billion and 16 billion years old, significantly older than the age of the universe determined by Baade. Astronomer Allan Sandage, who as a graduate student was an assistant to Hubble, came along and said you needed two things to make the universe that old: it had to be low enough in density to make a vast expansion, and the expansion rate had to be low.

Dueling Hubble constants

This, Siegel said, was where the controversy came in. Sandage said the expansion rate would have to be between 50-60km/sec/Mps. Rival astronomer Gérard de Vaucouleurs of France put it at around 100km/sec/Mpc. The race was on to make observations to see which group was right. Amazingly enough, each group’s observations matched up with what they thought the answer would be.

“This just goes to show that you cannot have the same people making the same measurements and trust them,” Siegel said. “This is why you need independent confirmation.”

It turns out Sandage and de Vaucouleurs were both wrong. There’s still no agreement on the right answer, but the disagreements are getting closer together. Sigel said the Hubble Space Telescope’s improvements in measuring the size of the universe return a value of 74±2km/sec/Mpc. The Planck mission’s observations of the cosmic microwave background radiation suggest 67±1km/sec/Mpc.

“There is a fight over the results like there always seems to be, because we are scientists and we cannot agree on anything,” Siegel said. “That is good, because questioning is what keeps us moving forward and what keeps us learning more.”

“The way we’re going to get there is with more and better data,” he added.

Better data

The better data will come from missions such as the European Space Agency’s Gaia, the James Webb Space Telescope, WFIRST, and the Large Synoptic Survey Telescope, which combined might improve our parallax measurements of cosmic distances by a factor of ten. We might also weed out faulty assumptions in the earlier work or get more accurate insights into the balance between matter and dark energy in the universe.

“If we can wait until the next decade, we might see that 74 number come down, we might also see the 67 number come up,” Siegel said. “The point is uncertainties are going to be reduced by more and better data.”

Siegel said that right now it’s pretty much agreed that the universe is about 13.8 billion years old and consists of about 30 percent matter and 70 percent dark energy. But the miniscule pluses or minuses can lead to huge fights.

“When that data comes in at last we will know exactly how fast our universe is expanding, how old it is, and what it all means for both our cosmic origins and our cosmic fate,” Siegel concluded. “That’s pretty good stuff.”


In the podcast linked below Siegel covers much of the topic matter of this article and his talk. His new book, Treknology: The Science of Star Trek from Tricorders to Warp Drive (Voyageur Press, 2017), is scheduled for release in October.

Share

4 thoughts on “The expanding universe: discovery, controversies, and hope

  1. Marc Carter

    I have seen estimates (on Wikipedia) that the universe is some 90 billion light years in diameter. It seems to me, being an amateur astronomer with no claim to special insights or great understandings, that if the universe is approximately 14 billion years old even if the expansion of the universe was at the speed of light (which makes an odd kind of sense to me) the universe expanding outward in 3 dimensions at that speed could be no larger than 28 billion light years in diameter (assuming a kind of spherical shape.) Is it possible that our flat but curved spacetime universe is a sphere in shape? If not, why would it not be so? Spherical form seems to be a universal constant. Can you define km/sec/Mpc? Is that km/sec/mega parsec? Are there really any measurements of the expansion of spacetime itself? Or is it just guesswork based on redshifts? Also, and I hope this doesn’t make you squirm even more, is it possible that light (EM waves) could experience time dilation effects? If not, why?

  2. Marc Carter

    To continue and insert further guesswork into my previous question I’d like to ask if there has been experimental work done to further verify Hubble’s redshift calculations. I find it hard to accept the idea that distant galaxies billions of light years away might move away spatially (spacetime being an odd duck for me to fully understand the dynamics of) from our galaxy at near (.8) the speed of light. Is it possible that light diffusion might add to the redshifts of distant galaxies in ways not previously measured? What about moving away in time more so than in space as a function of distance? Also, what about quantum fields and vacuum energies (among others, i.e. virtual particle interactions) permeating our universe as well as the bending and slowing of light due to massive bodies which the light from distant galaxies may pass through including dark matter? Might these factors also increase redshifts of distant galaxies?
    What if the first original stars (or objects which gave off EM waves) which came into being after inflation and the primordial (non-starlit) time ended have caused the expansion of the universe at the speed of light by sending out light (EM waves) in 4D cones at that time ? Perhaps the CMB is only a later layer of very large and periodic bursts of energy from those early times and an earlier wave is causing the expansion of spacetime (the universe.)
    Please feel free to shoot any of this down, I won’t be offended (as long as you are scientific about it) I make no claims to genius or complete comprehension of all the available facts on these subjects. I’m just a guy with funny ideas and perhaps ignorant questions. Being ignored is perhaps the worst of all.

  3. Greg Scheiderer Post author

    Marc,

    You raise interesting questions. Like you, I’m just an amateur astronomer with no claim to special insights or great understandings–I just have a blog! At least one of your questions I CAN answer. Yes, km/sec/Mpc is kilometers per second per megaparsec.

    It seems that I’ve heard some speaker or another postulate that distant stuff may be receding from us at greater than the speed of light, and this wouldn’t be breaking the law because the expansion of spacetime isn’t necessarily under this speed limit.

    Perhaps Dr. Siegel could take these on. You can find him at http://scienceblogs.com/startswithabang/author/esiegel/ and I’ll call his attention to this comment thread.

Comments are closed.