At the Largest Scales, Our Milky Way Galaxy is in the Middle of Nowhere

The Millenium Simulation created this image of the large-scale structure of the Universe, showing filaments and voids within the cosmic structure. According to a new study from the University of Wisconsin, our Milky Way is situated in a huge void in the cosmic structure. The Millennium Simulation is a project of the Max Planck Supercomputing Center in Germany. Image: Millennium Simulation Project

Ever since Galileo pointed his telescope at Jupiter and saw moons in orbit around that planet, we began to realize we don’t occupy a central, important place in the Universe. In 2013, a study showed that we may be further out in the boondocks than we imagined. Now, a new study confirms it: we live in a void in the filamental structure of the Universe, a void that is bigger than we thought.

In 2013, a study by University of Wisconsin–Madison astronomer Amy Barger and her student Ryan Keenan showed that our Milky Way galaxy is situated in a large void in the cosmic structure. The void contains far fewer galaxies, stars, and planets than we thought. Now, a new study from University of Wisconsin student Ben Hoscheit confirms it, and at the same time eases some of the tension between different measurements of the Hubble Constant.

The void has a name; it’s called the KBC void for Keenan, Barger and the University of Hawaii’s Lennox Cowie. With a radius of about 1 billion light years, the KBC void is seven times larger than the average void, and it is the largest void we know of.

The large-scale structure of the Universe consists of filaments and clusters of normal matter separated by voids, where there is very little matter. It’s been described as “Swiss cheese-like.” The filaments themselves are made up of galaxy clusters and super-clusters, which are themselves made up of stars, gas, dust and planets. Finding out that we live in a void is interesting on its own, but its the implications it has for Hubble’s Constant that are even more interesting.

Hubble’s Constant is the rate at which objects move away from each other due to the expansion of the Universe. Dr. Brian Cox explains it in this short video.

The problem with Hubble’s Constant, is that you get a different result depending on how you measure it. Obviously, this is a problem. “No matter what technique you use, you should get the same value for the expansion rate of the universe today,” explains Ben Hoscheit, the Wisconsin student who presented his analysis of the KBC void on June 6th at a meeting of the American Astronomical Society. “Fortunately, living in a void helps resolve this tension.”

There are a couple ways of measuring the expansion rate of the Universe, known as Hubble’s Constant. One way is to use what are known as “standard candles.” Supernovae are used as standard candles because their luminosity is so well-understood. By measuring their luminosity, we can determine how far away the galaxy they reside in is.

Another way is by measuring the CMB, the Cosmic Microwave Background. The CMB is the left over energy imprint from the Big Bang, and studying it tells us the state of expansion in the Universe.

This is a map of the observable Universe from the Sloan Digital Sky Survey. Orange areas show higher density of galaxy clusters and filaments. Image: Sloan Digital Sky Survey.
This is a map of the observable Universe from the Sloan Digital Sky Survey. Orange areas show higher density of galaxy clusters and filaments. Image: Sloan Digital Sky Survey.

The two methods can be compared. The standard candle approach measures more local distances, while the CMB approach measures large-scale distances. So how does living in a void help resolve the two?

Measurements from inside a void will be affected by the much larger amount of matter outside the void. The gravitational pull of all that matter will affect the measurements taken with the standard candle method. But that same matter, and its gravitational pull, will have no effect on the CMB method of measurement.

“One always wants to find consistency, or else there is a problem somewhere that needs to be resolved.” – Amy Barger, University of Hawaii, Dept. of Physics and Astronomy

Hoscheit’s new analysis, according to Barger, the author of the 2013 study, shows that Keenan’s first estimations of the KBC void, which is shaped like a sphere with a shell of increasing thickness made up of galaxies, stars and other matter, are not ruled out by other observational constraints.

“It is often really hard to find consistent solutions between many different observations,” says Barger, an observational cosmologist who also holds an affiliate graduate appointment at the University of Hawaii’s Department of Physics and Astronomy. “What Ben has shown is that the density profile that Keenan measured is consistent with cosmological observables. One always wants to find consistency, or else there is a problem somewhere that needs to be resolved.”

Profile of a Lonely Galaxy

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The vast majority of galaxies exist in clusters. These clusters are joined on larger scales by filaments and sheets of galaxies, between which, gigantic galactic voids are nearly entirely free of galaxies. These voids are often hundreds of million of light years across. Only rarely does a lonely galaxy break the emptiness. Our own Milky Way rests in one of these large sheets which borders the Local Void which is nearly 200 million light years across. In that emptiness, there have been tentative identifications of up to sixteen galaxies, but only one has been confirmed to actually be at a distance that places it within the void.

This dwarf galaxy is ESO 461-36 and has been the target of recent study. As expected of galaxies within the void, ESO 461-36 is exceptionally isolated with no galaxies discovered within 10 million light years.

What is surprising for such a lonely galaxy is that when astronomers compared the stellar disc of the galaxy with a mapping of hydrogen gas, the gas disc was tilted by as much as 55°. The team proposes that this may be due to a bar within the galaxy acting as a funnel along which gas could accrete onto the main disc. Another option is that this galaxy was recently involved in a small scale merger. The tidal pull of even a small satellite could potentially draw the gas into a different orbit.

This disc of gas is also unusually extended, being several times as large as the visual portion of the galaxy. While intergalactic space is an excellent vacuum, compared to the space within voids it is a relatively dense environment. This extreme under-density may contribute to the puffing up of the gaseous disc, but with the rarity of void galaxies, there is precious little to which astronomers can compare.

Compared with other dwarf galaxies, ESO 461-36 is also exceptionally dim. To measure brightness, astronomers generally use a measure known as the mass to light ratio in which the mass of the galaxy, in solar masses, is divided by the total luminosity, again using the Sun as a baseline. Typical galaxies have mass to light ratios between 2 and 10. Common dwarf galaxies can have ratios into the 30’s. But ESO 461-36 has a ratio of 89, making it among the dimmest galaxies known.

Eventually, astronomers seek to discover more void galaxies. Not only do such galaxies serve as interesting test beds for the understanding of galactic evolution in secular environments, but they also serve as tests for cosmological models. In particular the ΛCDM model predicts that there should be far more galaxies scattered in the voids than are observed. Future observations could help to resolve such discrepancies.

Structure of the Universe

[/caption]The large-scale structure of the Universe is made up of voids and filaments, that can be broken down into superclusters, clusters, galaxy groups, and subsequently into galaxies. At a relatively smaller scale, we know that galaxies are made up of stars and their constituents, our own Solar System being one of them.

By understanding the hierarchical structure of things, we are able to gain a clearer visualization of the roles each individual component plays and how they fit into the larger picture. For example, if we go down to the world of the very small, we know that molecules can be chopped down into atoms; atoms into protons, electrons, and neutrons; then the protons and neutrons into quarks and so on.

But what about the very large? What is the large-scale structure of the universe? What exactly are superclusters and filaments and voids? Let’s start by looking at galaxy groupings and move on to even larger structures.

Although there are some galaxies that are found to stray away by their lonesome, most of them are actually bundled into groups and clusters. Groups are smaller, usually made up of less than 50 galaxies and can have diameters up to 6 million light-years. In fact, the group in which our Milky Way is a member of is made up of only a little over 40 galaxies.

Generally speaking, clusters are bunches of 50 to 1,000 galaxies that can have diameters of up to 2-10 megaparsecs. One very peculiar property of clusters is that the velocities of their galaxies are supposed to be too high for gravity alone to keep them bunched together … and yet they are.

The idea that dark matter exists starts at this scale of structure. Dark matter is believed to provide the gravitational force that keeps them all bunched up.

A great number of groups, clusters and individual galaxies can come together to form the next larger structure – superclusters. Superclusters are among the largest structures ever to be discovered in the universe.

The largest single structure to be identified is the Sloan Great Wall, a vast sheet of galaxies that span a length of 500 million light-years, a width of 200 million light-years and a thickness of only 15 million light-years.

Due to the limitations of today’s measuring devices, there is a maximum level to which we can zoom out. At that level, we see a universe made up of mainly two components. There are the threadlike structures known as filaments that are made up of isolated galaxies, groups, clusters and superclusters. And then there are vast empty bubbles of empty space called voids.

You can read more about structure of the universe here in Universe Today. Want to read about the cosmic void: could we be in the middle of it? We’ve also written about probing the large scale structure of the universe.

There’s more about it at NASA. Here are a couple of sources there:

Here are two episodes at Astronomy Cast that you might want to check out as well:

Sources: NASA WMAP, NASA: Sheets and Voids