Impossibly Aligned Galaxies

Astronomers have found an alignment of galaxies along our line of sight that’s so rare, it ought to be impossible. Is it just luck — or does it tell us something more?

Galaxy cluster with giant arc
A montage shows where the arc lies relative to the galaxy cluster.
NASA, ESA, A. Gonzalez (Univ. of Florida), A. Stanford (UC Davis and Lawrence Livermore National Laboratory), and M. Brodwin (Univ. of Missouri and Harvard-Smithsonian Center for Astrophysics)
The Hubble Space Telescope image shows it clearly: a giant blue arc nestled among a massive cluster of galaxies. Light from a distant, large, spiral-type background galaxy was warped into an arc as it passed the massive cluster en route to Earth. Einstein predicted such gravitational light-bending in 1916, and with the advent of Hubble, warped images of extremely far galaxies have become a relatively common sight. But there’s a problem with this one: it shouldn’t exist.

“When I first saw it, I kept staring at it, thinking it would go away,” says lead scientist Anthony Gonzalez (University of Florida). “According to a statistical analysis, arcs should be extremely rare at that distance.”

The problem is not that the background galaxy exists. Though its distance is not well known, we’re probably seeing it from when the universe was between 0.7 and 3.7 billion years old. The first galaxies formed when the universe was less than about 1 billion years old. So the galaxy is young, but not impossibly young.

Nor is the problem the existence of such a massive galaxy cluster. It’s a big ’un though. Weighing as much as 500 trillion Suns, it’s five to 10 times more massive than other clusters its age. That puts it on track to become one of the most massive clusters in the present-day universe, with as much mass as the Coma or El Gordo galaxy clusters.

“The galaxy-cluster community has been calling these [massive clusters] pink elephants, and this guy’s the pinkest,” notes Daniel Stern (JPL), a co-author of the study.

Still, the existence of such a massive cluster in a universe only 4 billion years old might be rare, but it’s not impossible.

Giant arc
The light from a distant background galaxy was distorted into a giant, fuzzy arc as it passed by the massive galaxy cluster en route to Earth. The background galaxy is blue, which means it hosts forming stars, so its original shape may have been spiral.
NASA, ESA, A. Gonzalez (Univ. of Florida), A. Stanford (UC Davis and Lawrence Livermore National Laboratory), and M. Brodwin (Univ. of Missouri and Harvard-Smithsonian Center for Astrophysics)
The almost-impossibility lies in the chance alignment of such a large background galaxy and such a large foreground cluster. The odds of there being enough of both are small. So small, in fact, that Gonzalez and his co-authors write in their paper, “This arc simply should not exist” (their emphasis).

One possible explanation requires a change to the earliest moments of the Big Bang. The current model says that the newborn universe went through a super-expansion, or “inflation,” that lasted only from 10-36 to 10-33 seconds. During that moment, inflation enlarged microscopic quantum fluctuations enough that they became the gravitational seeds for the formation of galaxies. The physics all works out beautifully.

The question is, what exactly did these seeds look like? Some versions of inflation produce seeds that tend to be on the large side. Larger seeds would make large, star-forming galaxies and massive galaxy clusters more common in the early universe. If that were true, the chance alignment of a large galaxy behind a massive cluster becomes more likely.

Exotic physics in the early universe make astronomers uneasy. But other explanations don’t work well. For example, Gonzalez points out that young galaxy clusters may have been more concentrated in their centers. That would enable clusters to distort light from more background galaxies, making arcs like this one more common. But they still wouldn’t be common enough — the odds of finding a giant arc at such a great distance would still be near zero.

“I’m not yet convinced by any of these explanations,” says Gonzalez. “After all, we have found only one example.” But it should spur the search for more.

Monica Young

About Monica Young

Monica Young, a professional astronomer by training, is web editor of Sky & Telescope, where she creates, manages, and maintains website content, and contributes to the magazine.

12 thoughts on “Impossibly Aligned Galaxies

  1. George Brewer

    What one should consider is what happens after Planck time, as I like to call it. The numerical value of Planck’s constant as milestone in the early universe. These two rare instances are possible. It is only with the advent of "better seeing", we are able to detect the menagerie that composes the universe.

  2. Rod

    "Exotic physics in the early universe make astronomers uneasy." This statement has much to say about the postulated inflation epoch in the big bang model. We need to remember issues like the magnetic monopoles created in the big bang that inflation pushes into some other universe (Guth, A. H. and Steinhardt, P. J., June 1984 Scientific American, The Inflationary Universe). We also have the ‘inflaton’ particle that initially is a repulsive gravitational force that decays somehow and we end up with the present gravitational force that is attractive. Now this report shows there are different types of seeds that emerge from inflation where the galaxies could form, but clearly different seeds will yield different galaxy arrangements too. Okay I will stop here, smile.

  3. Mt. Nipomo Observatory (aka:Foggy Meas)

    Perhaps this arc is a remnant of Inflation’s shock wave. Or compressed gas/mass/plasma wavefront segment/remnant seen obliquely, like Veil nebulae, only HUGE (dude.)
    Clear skies

  4. Sonia Sullivan

    I disagree with all of you. A "Universe" can not inflate or contract, into another Universe. That Black stuff we see out there goes on and on. Planets and Stars fill the void. Sometimes, things Are exactly as we see them!

  5. Bryan

    Well Sonia, things are not exactly as we see them. Probably more like they used to be. Remember, you have to consider that time/speed of light "thingy". Just saying

  6. Rod

    Sonia et al. The CMBR is the limit of our observable universe according to the standard big bang model. Beyond the CMBR the universe could exist quite differently than what we see including matter. This report briefly touched upon the inflation epoch in big bang as a possible answer for the giant arc. Strange things happen when you extend the big bang model back to the Planck time of 10E-44 second after the big bang. In the late 1990s, Neil Turok and Stephen Hawking developed metrics where the pea instanton particle creates the universe from the cosmological singularity that arises when you extrapolate back to the Planck time of 10E-44 second after big bang. Extrapolating from the present back to the big bang using Einstein GR results in the cosmological singularity for the origin of the universe. The cosmological singularity cannot explain the origin of physical law in nature. Today cosmologists may argue for the multi-verse arising so many different universes with many different laws could exist along with many big bang events going on forever, thus the universe we see is just one of an infinite number of universes. I personally accept Genesis 1:1 as the correct answer for the origin of the universe.

  7. Bruce

    That’s very well stated Rod. I agree that theories leaving a Creator out of the mix “cannot explain the origin of physical law in nature.” It seems purely logical to me that the existence of law implies the existence of a Lawgiver. This “multiverse” notion just seems like grasping at straws in an attempt to brush away the exquisitely fine tuning of the physical laws that make the universe and organic life inside it possible. There’s a huge anti divine origins bias in the sciences today, but it’s good to remember that biases are like binders that cause people to overlook evidence that contradicts their viewpoint.

  8. Garry

    What is meant by the term "multiple universes"? The dictionary definition of universe is "The whole body of things and phenomena observed or postulated." The above defintion seems to allow two possibilities: 1) the universe is everything we can observe, or 2) the universe is everything we can postulate. The first case leaves open the possibility of other unobservable "universes". The second case means there is only one universe, which includes other systems which can be postulated. In the second case, everything which can be postulated belongs to this universe, regardless of whether we can observe all of it. I suppose the term "multiple universes" uses the first definition, making a distinction between another postulated system that cannot be observed from the system in which we reside?

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