Expansion Explained

Expansion Explained[1]

Q: How did astronomers discover that the expansion of the universe was accelerating?

(A question from Steve Ciucci, Madison, Wisconsin)

A: In the early 1990s, two teams of astronomers set out to measure the universe’s expansion history to predict its future. If the universal expansion was slowing down a lot, it would someday stop and reverse itself, ending in a hot Big Crunch. If, instead, the deceleration was small, the universe would expand forever (albeit at a decelerating rate).

Each of these possibilities predicted a different relationship between the distances (or, more precisely, “lookback times”) of galaxies and their redshifts (the amount the universe had stretched while the light was on its way to us).

Redshifts are easy to measure from spectra of galaxies, except when the galaxies are faint. Astronomers obtain distances of nearby galaxies by finding a star of known luminosity (power, or absolute magnitude), measuring its apparent brightness (apparent magnitude), and applying the inverse-square law of light. But at great distances, normal stars can’t be seen, so the astronomers used Type Ia supernovae—exploding stars resulting from white dwarfs that approach their maximum possible mass (the Chandrasekhar limit). These can be billions of times as powerful as the Sun, and their peak luminosities are nearly uniform.

The two teams used large telescopes to take deep images of various parts of the sky, repeating these same fields a few weeks later. They found supernova candidates among the thousands of faint galaxies. They then obtained spectra of the candidates to confirm that they were Type Ia supernovae and to obtain their redshifts. By repeatedly imaging these supernovae, they measured their light curves and peak brightnesses. The teams also took issues into account such as the nonuniformity of Type Ia supernovae, the presence of intervening dust, and possible cosmic evolution of supernovae.

The result was that the supernovae were too faint (for a given redshift) to be consistent with decelerating or constant-speed expansion of the universe. Instead, the data implied that the expansion has been accelerating in the past 5 billion years. (Later measurements revealed the era of deceleration during the first 9 billion years.) Other techniques have now confirmed this acceleration, which most astrophysicists attribute to the presence of dark energy of unknown origin.

[1] From AskAstro (Alex Fillppenko, (Professor of Astronomy, University of California, Berkeley, CA), in Astronomy (44, 6, April 2016, p. 34)

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New Map of the Universe

Astronomers Make The Largest Map of The Universe Yet[1]

The image at left shows a slice through largest-ever three-dimensional map of the Universe. Earth is at the left, and distances to galaxies and quasars are labelled by the lookback time to the objects (lookback time means how long the light from an object has been traveling to reach us here on Earth).

The locations of quasars (galaxies with supermassive black holes) are shown by the red dots, and nearer galaxies mapped by SDSS are also shown (yellow). The right-hand edge of the map is the limit of the observable Universe, from which we see the Cosmic Microwave Background (CMB)–the light “left over” from the Big Bang. The bulk of the empty space in between the quasars and the edge of the observable universe are from the “dark ages”, prior to the formation of most stars, galaxies, or quasars. [Credit: Anand Raichoor (École polytechnique fédérale de Lausanne, Switzerland) and the SDSS collaboration.]

Astronomers with the Sloan Digital Sky Survey (SDSS) have created the first map of the large-scale structure of the Universe based entirely on the positions of quasars. Quasars are the incredibly bright and distant points of light powered by supermassive black holes.

“Because quasars are so bright, we can see them all the way across the Universe,” said Ashley Ross of the Ohio State University, the co-leader of the study. “That makes them the ideal objects to use to make the biggest map yet.”

The amazing brightness of quasars is due to the supermassive black holes found at their centers. As matter and energy fall into a quasar’s black hole, they heat up to incredible temperatures and begin to glow. It is this bright glow that is detected by a dedicated 2.5-meter telescope here on Earth.

“These quasars are so far away that their light left them when the Universe was between three and seven billion years old, long before the Earth even existed,” said Gongbo Zhao from the National Astronomical Observatories of Chinese Academy of Sciences, the study’s other co-leader.

To make their map, scientists used the Sloan Foundation Telescope to observe an unprecedented number of quasars. During the first two years of the SDSS’s Extended Baryon Oscillation Spectroscopic Survey (eBOSS), astronomers measured accurate three-dimensional positions for more than 147,000 quasars.

The telescope’s observations gave the team the quasars’ distances, which they used to create a three-dimensional map of where the quasars are. But to use the map to understand the expansion history of the Universe, they had to go a step further, using a clever technique involving studying “baryon acoustic oscillations” (BAOs). BAOs are the present-day imprint of sound waves which travelled through the early Universe, when it was much hotter and denser than the universe we see today. But when the universe was 380,000 years old, conditions changed suddenly and the sound waves became “frozen” in place. These frozen waves are left imprinted in the three-dimensional structure of the Universe we see today.

The good news about these frozen waves—the original baryon acoustic oscillations—is that the process that produced them is simple. Thus, we have a good understanding of what BAOs must have looked like at that ancient time. When we look at the three-dimensional structure of the Universe today, it contains these same BAOs grown out to a huge scale by the expansion of the Universe. The observed size of the BAO can be used as a “standard ruler” to measure distances. Just as by using the apparent angle of a meter stick viewed from the other side of a football field, you can estimate the length of the field. “You have meters for small units of length, kilometers or miles for distances between cities, and we have the BAO scale for distances between galaxies and quasars in cosmology,” explained Pauline Zarrouk, a PhD student at the Irfu/CEA, University Paris-Saclay, who measured the projected BAO scale.

Astronomers from the SDSS have previously used the BAO technique on nearby galaxies and then on intergalactic gas distributions to push this analysis farther and farther back in time. The current results cover a range of times where they have never been observed before, measuring the conditions when the universe more than two billion years before the Earth formed.

The results of the new study confirm the standard model of cosmology that researchers have built over the last twenty years. In this standard model, the Universe follows the predictions of Einstein’s general theory of relativity—but includes components whose effects we can measure, but whose causes we do not understand. Along with the ordinary matter that makes up stars and galaxies, the universe includes dark matter—invisible yet still affected by gravity—and a mysterious component called “dark energy.” Dark energy is the dominant component at the present time, and it has special properties that cause the expansion of the universe to speed up.

“Our results are consistent with Einstein’s theory of general relativity,” said Hector Gil-Marin, a researcher from the Laboratoire de Physique Nucléaire et de hautes Énergies in Paris who undertook key parts of the analysis. “We now have BAO measurements covering a range of cosmological distances, and they all point to the same thing: the simple model matches the observations very well.”

Even though we understand how gravity works, we still do not understand everything—there is still the question of what exactly dark energy is. “We would like to understand dark energy further,” said Will Percival from the University of Portsmouth, who is the eBOSS survey scientist. “Surveys like eBOSS are helping us to build up our understanding of how dark energy fits into the story of the universe.”

The eBOSS experiment is still continuing, using the Sloan Telescope at Apache Point Observatory in New Mexico, USA. As astronomers with eBOSS observe more quasars and nearby galaxies, the size of their map will continue to increase. After eBOSS is complete, a new generation of sky surveys will begin, including the Dark Energy Spectroscopic Instrument (DESI) and the European Space Agency Euclid satellite mission. These will increase the fidelity of the maps by a factor of ten compared with eBOSS, revealing the universe and dark energy in unprecedented detail.

[1] “Astronomers Make The Largest Map of The Universe Yet,” Astronomy Now (22 May 2017)

Expanding Universe

Universe Expanding Faster than Expected[1]

Astronomers trying to pinpoint the Hubble constant—the rate at which the universe is expanding—found themselves with a number 5 to 9 percent faster than previously thought.

Measurements of the Hubble constant have varied hugely since Edwin Hubble first observed that the universe appears to be retreating in all directions. By observing the fading glow of the Big Bang, researchers with the Planck mission uncovered on number for the Hubble constant. But in general, Hubble constant queries rely on knowing an object’s distance combined with how quickly it’s receding. And distance in the universe is surprisingly tricky to pin down.

For the nearby universe, Cepheid variable stars (shown in the image above: Cepheid variable stars are in the red circles) can reveal distances. They change brightness at regular time intervals, and the length of this period is directly related to their intrinsic brightness. For the less local universe, astronomers use type Ia supernovae, which all light up with the same brightness. In both cases by measuring how bright the objects appear, astronomers can calculate the intervening distance between them and Earth.

Now, astronomers led by Nobel laureate Adam Riess have used the Hubble and Keck observatories to combine these Cepheid and supernova distances into one highly accurate number.

They’ve done the world’s best job of decreasing the uncertainty in the measured rate of universe expansion and of accurately assessing the size of this uncertainty, yet they found that their measured rate of expansion is probably incompatible with the rate expected from observations of the young universe, suggesting that there’s something important missing in our physical understanding of the universe.

 

[1] See Korey Haynes, “Universe Expanding Faster than Expected,” Astronomy (44, 10, October 2016, p.11).

The Missing Universe

The Missing Universe[1]

When you look up at the night sky, do you feel that something is missing? If you don’t, you should. Eighty-one years ago, astronomers discovered that much of the cosmos’ contents was invisible. They saw extra gravity pulling material around galactic centers, but they found no extra mass to account for that force. They deemed their conundrum the “missing mass problem” and came up with a partial solution: dark matter.

Dark matter, an invisible substance of unknown quality, character, and origin that permeates the universe, neither emits nor reflects light. But it makes up 85 percent of the cosmos’ mass. Or does it? The idea has some detractors, who say that the problem is not “missing mass” but a misunderstanding of gravity itself: Scientists need to modify Newton’s original ideas, and then the mathematical need to understand the universe via mysterious, elusive dark matter will disappear.

Above is a picture of the Whirlpool Galaxy (Messier 51). Like all other spiral galaxies, it seems to possess about 10 times more gravity than its visible contents can account for. Either dark matter surrounds these galaxies, or gravity olpe4rates differently than most scientists think.

As far as we know, matter exists in five (observable) states: plasma, gas, liquid, solid, or Bose-Einstein condensate. Quarks combine into protons and neutrons to form atomic nuclei held together by the strong force, and these can join with electrons into atoms held together by the electromagnetic force. A cloud of electrically charged atomic nuclei (ions) and electrons is a plasma. When atoms (or molecules) float loose, they form a gas. When atoms attach weakly to neighbors, they form matter’s rarest state in the universe, a liquid. When atoms rigidly bond to neighbors, they constitute a solid. At temperatures colder than any natural place in the universe, atoms can merge spookily into a single state, the Bose-Einstein condensate.

Dark matter would be a new form. Is there evidence? Perhaps. A galaxy’s rotation curve (compared above) compares the velocities of stars in the disk with their distances from the galaxy’s center. Observed curves appear flat, suggesting that a halo of unseen matter or gravity behaves differently in weak fields. Without dark matter or Modified Newtonian Dynamics, the curve would drop off at increasing distance.

Until dark matter particles can be confirmed by observation, astronomers can only keep gazing into the night sky, conducting experiments, and wondering whether most of the cosmos is really missing.

[1] See Bob Berman, “The Missing Universe,” Astronomy (42, 4, April 2014)

Runaway Universe

Runaway Universe[1]

There is a crisis brewing in the cosmos, or perhaps in the community of cosmologists. The universe seems to be expanding too fast, some astronomers say.

Recent measurements of the distances and velocities of faraway galaxies don’t agree with a hard-won “standard model” of the cosmos that has prevailed for the past two decades.

The latest result shows a 9 percent discrepancy in the value of a long-sought number called the Hubble constant, which describes how fast the universe is expanding. But in a measure of how precise cosmologists think their science has become, this small mismatch has fostered a debate about just how well we know the cosmos.

“If it is real, we will learn new physics,” said Wendy Freedman of the University of Chicago, who has spent most of her career charting the size and growth of the universe.

The Hubble constant, named after Edwin Hubble, the Mount Wilson and Carnegie Observatories astronomer who discovered that the universe is expanding, has ever given astronomers fits. In an expanding universe, the farther something is away from you, the faster it is receding. Hubble’s constant tells by how much.

But measuring it requires divining the distances of lights in the sky—stars and even whole galaxies that we can never visit or recreate in the lab. The strategy since Hubble’s day has been to find so-called standard candles, stars or whole galaxies whose distances can be calculated by how bright they look from Earth.

But the calibrators themselves need to be calibrated, which has led to a rickety chain of assumptions and measurements in which small errors and disagreements—about, say, how much dust is interfering with observations—can build up to cosmic proportions. Only three decades ago, renowned astronomers could not agree on whether the universe was 10 billion or 20 billion years old. Now everybody has settled on its age as about 13.8 billion years.

Using a new generation of instruments like the Hubble Space Telescope, astronomers have steadily whittled down the uncertainty in the Hubble constant.

In 2001, a team led by Dr. Freedman reported a value of 72 kilometers per second per megaparsec (about 3.3 million lightyears), in the galumphing units astronomers prefer. It meant that for every 3.3 million lightyears a galaxy was farther away from us, it was moving 72 kilometers a second faster.

Hubble’s original estimate was much higher, at 500 in the same units of measurement.

Dr. Freedman’s result had an error margin that left it happily consistent with other more indirect calculations, that had gotten a slightly slower and lower value of 67 for the Hubble constant. Those were derived from studies of microwaves emitted and still lingering in the sky from the primordial Big Bang fireball.

As a result, in recent years, astronomers have settled on a recipe for the universe that is as black and as decadent as a double dark chocolate chunk brownie. The universe consists of roughly 5 percent atomic matter by weight, 27 percent mysterious dark matter and 68 percent of the even more mysterious dark energy that is speeding up the cosmic expansion. Never mind that we don’t know exactly what all this dark stuff is. Astronomers have a good theory about how it behaves, and that has allowed them to tell a plausible story about how the universe evolved from when it was a trillionth of a second old until today.

But now the Hubble precision has gotten seemingly better, and the universe might be in trouble again.

Last summer a team led by Adam Riess of Johns Hopkins University and the Space Telescope Science Institute, using the Hubble Space Telescope and the giant Keck Telescope on Mauna Kea in Hawaii and supernova explosions as the ultimate distance markers, got a value of 73 plus or minus only 2.4 percent for the elusive constant.

That made waves because it meant that, if true, the Hubble constant as observed today was now clearly incompatible with a result of the lower slower value of 67 inferred from data obtained in 2013 by the European Planck spacecraft of relic radiation from the Big Bang. The Planck mission observations that show the universe when it was only 380,000 years old are considered the gold standard of cosmology.

Whether the standard cosmic recipe might now need to be modified—for example, to account for a new species of subatomic particles streaming through space from the Big Bang—depends on whom you talk to. Some say it is too soon to get excited about new physics sneaking through such a small discrepancy in a field noted for controversy. With more data and better understanding of statistical uncertainties, the discrepancy might disappear, they say.

“No explanation I know of is less ugly than the problem,” Lawrence M. Krauss, a theorist at Arizona State, said.

Others say this could be the beginning of something big. David Spergel, a cosmologist at Princeton and the Simons Foundation, called the discrepancy “very intriguing,” but said he was not yet convinced that this was the signature of new physics. Michael S. Turner of the University of Chicago said, “If the discrepancy is real, this could be a disruption of the current highly successful standard model of cosmology and just what the younger generation wants—a chance for big discoveries, new insights and breakthroughs.”

Dr. Riess and his colleague Stefano Casertano got roughly the same answer of 73 later last summer, strengthening the claim for a mismatch of Hubble constants. They used early data from the European spacecraft GAIA, which is measuring the distances of more than a billion stars by triangulation, thus allowing astronomers to skip some of the lower rungs on the distance ladder.

They calculated that the odds of this mismatch being a statistical fluke were less than one part in a hundred—which might sound good in poker but not in physics, which requires odds of less than one in a million to cement a claim of a discovery.

“I think it’s a potentially serious issue,” said Alex Filippenko, a University of California astronomer who is part of the team. “In this line of research the devil is in the details. And after getting the details right, we’re left with a major puzzle.”

George Efstathiou, of the University of Cambridge and one of the leaders of the Planck mission responsible for its cosmological analysis, said Dr. Riess and his team had underestimated the errors in their measurement.

“So, in summary, I think that the Planck results are secure,” he wrote in an email. “They,” he said, referring to the other astronomers, “may be right and we have to modify their standard model, but the evidence looks weak to me.”

Dr. Riess and his colleagues have stood by their work, however, and the plot thickened further in December when a group called H0LiCOW (don’t ask) from the Max Planck Institute for Astrophysics in Garching, Germany, reported its own value of 72 for the Hubble constant, also inconsistent with the Planck space mission’s analysis.

Led by Sherry Suyu of Max Planck, the group measured the delays experienced by light rays from five distant flickering quasars as they followed different paths around massive galaxies on the way from Out There to us. . The technique, they say, depends only on geometry and Einstein’s theory of gravity, general relativity, making it independent of other assumptions about dust or the makeup of stars.

Last year, a group known as BOSS, the Baryon Oscillation Spectroscopic Survey, came up with a Hubble constant of about 68, based on how 1.5 million galaxies were clustered in space and time, but it used data from the cosmic microwave background for calibration.

There is wiggle room, Dr. Riess and others say, for both the modern and the primordial results to be right, because Planck measures the Hubble constant only indirectly as one of several parameters in the standard model of the universe. Other parameters could be tweaked.

That is where new physics might come in.

The most likely candidates to fill the gap, Dr. Riess said, might be a new form of the ghostly particles called neutrinos, already known to be abundant in the cosmos. They come in three types that can change into one another as they traverse space; some physicists have suggested there could be a fourth kind, called sterile neutrinos, that don’t interact with anything at all.

Their discovery could unlock new realms in particle physics and perhaps shed light, so to speak, on the quest to understand the dark matter that suffuses space and provides the gravitational scaffolding for galaxies.

Another possibility is that the most popular version of dark energy—known as the cosmological constant, invented by Einstein 100 years ago and then rejected as a blunder—might have to be replaced in the cosmological model by a more virulent and controversial form known as phantom energy, which could cause the universe to eventually expand so fast that even atoms would be torn apart in a Big Rip billions of years from now.

“This is a very interesting tension,” Dr. Riess said. “This is why we play the game. We look for something not fitting.”

He added, “Clues about the dark sector or about fundamental physics are in play.”

This is the age of “precision cosmology,” and while everybody agrees that it is still too soon to tell, the avalanche of data from GAIA and the coming James Webb Space Telescope is just beginning, Dr. Freedman said. In the next few years she hopes the Hubble constant can be measured to 1 percent accuracy.

“And that’s what makes it interesting—this is feasible, and a lot of work is now ongoing that will allow us to resolve this within the next couple of years,” she said. “It’s what makes me want to work on this again!”

She said the situation reminded her of the late 1990s, when discrepant distances to distant supernova explosions led to the discovery that the expansion of the universe was accelerating under the influence of dark energy. Dr. Riess won a Nobel Prize for his part in that, and dark energy took its place in cosmic orthodoxy.

“It’s not quite ‘déjà vu,’” he wrote in an email, “but it’s funny that whenever my colleagues and I look at the contemporary universe with our radar guns, it’s expanding too fast for the contemporary expectations!”

[1] Dennis Overbye, “Cosmos Controversy: The Universe Is Expanding, but How Fast?” in The New York Times (Feb 20, 2017), pp. D1, D8. A version of this article appears in print on February 21, 2017, on Page D1 of the New York edition with the headline: A Runaway Universe. Downloaded March 1, 2017