Astronomy

How is the Milky Way on a collision course with Andromeda when the universe is expanding?

How is the Milky Way on a collision course with Andromeda when the universe is expanding?


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At some time in the future it is said our galaxy will collide with Andromeda. Yet at the same time the universe is expanding at a ever-accelerating pace. Using the old raisin-in-the-baking-loaf-of-bread analogy, the raisins grow farther apart and don't “collide” with each other during the baking process. It seems to be the same way with galaxies and the expanding universe.

How are these two seemingly conflicting postulates reconciled? What am I missing here?


I'm not an expert, but I looked into this recently when toying with the idea of a scale model of the observable universe. To the best of my understanding, the following is approximately correct.

TLDR: The expansion happens on the very large scale of the universe. At the scale of the observable universe the Andromeda galaxy is a next-door neighbour, and at the local scale gravity is strong enough to override any effect of expansion.

The Milky Way is 100,000 light years across, 2.5 million light years brings us to the Andromeda galaxy.

Galaxies cluster in galaxy groups, and 10 million light years span about 50 galaxies and brings us to the edge of what's called the Local Group.

Galaxy groups cluster into superclusters. We're part of the Laniakea Supercluster, which spans about 100,000 galaxies and 500 million light years.

For any appreciable expansion (redshift 0.1) you need at least a billion light years. It's something that happens in the voids between superclusters, and doesn't become really pronounced until you're halfway across the observable universe (redshift 1, 7-8 billion light years as measured in light travel time).

For the "raisins in a loaf" analogy, I think it's not too wrong if you think of the raisins as superclusters rather than individual galaxies. Note that although the dough expands in a baking loaf, individual raisins do not, because there are other forces at work that hold them together.


In general, space in the Universe is expanding. However, this expansion can be locally retarded by high enough densities of mass-energy. On very large scales, the mean density is low enough that you can treat the whole universe as expanding. But if you look on smaller scales, you can find underdense regions (voids) where the expansion is actually faster than average, and overdense regions where the expansion is slower or has even ceased. Inside of clusters and galaxy groups (and of course inside galaxies themselves), the density is high enough that space is no longer expanding, and so it's perfectly possible for galaxies to move through local space towards each other and collide.

So in your raisin-loaf model, the trick is that regions right around each raisin -- and in local regions containing several raisins close to each other -- aren't expanding.

Note that j-g-faustus' argument is somewhat incorrect: if expansion only occurs in voids between superclusters, then Edwin Hubble would never have found it by looking at galaxies within the nearest 30 million light years. Space inside individual clusters and groups is not expanding, but space within superclusters is (just not as fast as space within voids).


Just as molecules in the bread have random motion due to heat, galaxies have random motion due to variations in the initial state of the universe. For galaxies that are close by, those variations are smaller than the Hubble expansion, and the Hubble expansion is simply a general trend; object that are farther away from us tend to, on average, be moving away from us at higher speed, but individual objects can still be moving towards us.

According to this site, the Hubble constant is "around 70 kilometers per second for every megaparsec in distance". According to wikipedia, Andromeda is "780 kiloparsecs" away. Multiplying those together, we get that Andromeda's movement due to the Hubble expansion should be about 5 km/sec. Wikipedia also says that "The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second". That means that Andromeda's motion due to Hubble expansion is about 4% of its total movement. Andromeda has so much velocity relative to Earth that the Hubble expansion simply gets drowned out.


In an expanding universe, why are Andromeda and the Milky Way destined to collide?

The spiral galaxies of Arp 271 illustrating what will one day happen to Andromeda and our Milky Way. Image Source, Credit & Copyright: Gemini Observatory, GMOS-South, NSF.

If the Universe is expanding and all galaxies are moving apart from one another, why is it that the Milky Way and Andromeda will one day collide?

Thanks for the great question! To get to the answer, we will need to look at which forces operate on small and large astronomical scales (distances). The two fundamental forces at play, in this case, are gravity and dark energy. Gravity is certainly the more familiar of the two, so let’s start there.

On the smaller scales of the universe, gravity is the dominant force. We see gravity in action every day on Earth when we jump or drop our beloved phones, and it is famously responsible for causing an apple to hit Sir Isaac Newton in the head. These Earthly scales are actually some of the smallest scales on which gravity dominates. On slightly larger scales, gravity is in charge of holding together the solar system, the Milky Way galaxy, and all of the nearby galaxies (known as the Local Group) including Andromeda. On scales much larger than the size of the Local Group, the gravitational attraction between two galaxy-sized objects becomes dramatically weaker (Newton’s Law of Universal Gravitation is an inverse square law), and dark energy takes over. So, what is dark energy?

In the late 1920s, Edwin Hubble determined that the universe is expanding at an accelerating rate. This was a key discovery that even altered Albert Einstein’s view of the universe (check out Einstein’s “biggest blunder”). Astronomers call the mysterious force driving the universe apart “dark energy” (NASA). Unfortunately, we don’t know very much about dark energy beyond that. Hopefully, work done at the University of Texas at Austin and McDonald Observatory using the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) will provide us with further insights in the coming years.

Returning to your question, galaxy collisions happen on relatively small scales in an astronomical sense. The strong gravitational attraction between the Milky Way and Andromeda wins out over the dark energy attempting to drive them apart, and will ultimately cause a collision to occur. But don’t lose any sleep over our inevitable collision with Andromeda, it won’t happen for a few billion years!

Sydney Sherman


Why are the Milky Way and Andromeda Galaxies Moving Toward Each Other While the Universe is Expanding?

if everything is moving away from everything else, how we can they say the milky way and andromeda will someday colide?

Answer:

The Andromeda and Milky Way galaxies are moving toward each other due to mutual gravitational attraction. This mutual gravity force is stronger than the force which causes the expansion of the Universe on the relatively short distances between Andromeda and the Milky Way. This interaction between the Andromeda and Milky Way galaxies is actually quite common in the Universe, where we see many galaxies either in the process of interaction, or even collision, or showing evidence of past interaction. These more “local” interactions exist over relatively small size scales, in comparison to the size scales over which the expansion of the Universe is measured.


Our Milky Way galaxy is on collision course with nearby Andromeda galaxy

The galaxy where we live is called the Milky Way. As per the recent study that involves studying the past of neighboring large galaxy Andromeda, our milky way is on its collision course with Andromeda.

Andromeda has consumed several smaller galaxies, likely inside the last few billion years, with left-overs found in massive streams of stars. Now, it has set its sights on the Milky Way as its next main course.

ANU researcher Dr Dougal Mackey, who co-led the study with Professor Geraint Lewis from the University of Sydney, said, “The international research team also found very faint traces of more small galaxies that Andromeda gobbled up even earlier, perhaps as far back as 10 billion years when it was first forming.”

But, no need to get panic- It won’t happen for about 4.5 billion years.

Dr. Mackey, from the ANU Research School of Astronomy and Astrophysics, said, “The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way’s ultimate fate.”

“Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalized many more galaxies, possibly larger ones.”

“By tracing the faint remains of these smaller galaxies with embedded star clusters, we’ve been able to recreate the way Andromeda drew them in and ultimately enveloped them at different times.”

The disclosure introduces several new mysteries, with the two bouts of galactic feeding originating from totally different directions. However, the most surprising discovery in the direction of the ancient feeding is the same as the bizarre ‘plane of satellites’, an unexpected alignment of dwarf galaxies orbiting Andromeda.

Previously, scientists discovered that such planes were fragile and rapidly destroyed by Andromeda’s gravity within a few billion years.

Professor Lewis said, “This deepens the mystery as the plane must be young, but it appears to be aligned with ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation. We’re going to have to think quite hard to unravel what this is telling us.”

Dr. Mackey said, “studying Andromeda also informed understanding about the way our galaxy has grown and evolved over many billions of years.”

“One of our main motivations in studying astronomy is to understand our place in the Universe. A way of learning about our galaxy is to study others that are similar to it and try to understand how these systems formed and evolved.”

“Sometimes, this can be easier than looking at the Milky Way because we live inside it, and that can make certain types of observations quite difficult.”

The study, published in Nature, analyzed data from the Pan-Andromeda Archaeological Survey, known as PAndAS.


Astronomy Picture of the Day

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2012 June 4
Milky Way Galaxy Doomed: Collision with Andromeda Pending
Illustration Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), and A. Mellinger

Explanation: Will our Milky Way Galaxy collide one day with its larger neighbor, the Andromeda Galaxy? Most likely, yes. Careful plotting of slight displacements of M31's stars relative to background galaxies on recent Hubble Space Telescope images indicate that the center of M31 could be on a direct collision course with the center of our home galaxy. Still, the errors in sideways velocity appear sufficiently large to admit a good chance that the central parts of the two galaxies will miss, slightly, but will become close enough for their outer halos to become gravitationally entangled. Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years. Pictured above is an artist's illustration of the sky of a world in the distant future when the central parts of each galaxy begin to destroy each other. The exact future of our Milky Way and the entire surrounding Local Group of Galaxies is likely to remain an active topic of research for years to come.


Warning! Cannibalistic Andromeda Will Eat the Milky Way Alive…

Astronomers have pieced together the cannibalistic past of our neighboring large galaxy Andromeda, which has now set its sights on the Milky Way as its next main course.

“The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way’s ultimate fate,” said Dr. Mackey from the ANU Research School of Astronomy and Astrophysics.

The galactic detective work found that Andromeda has eaten several smaller galaxies, likely within the last few billion years, with left-overs found in large streams of stars.

ANU researcher Dr. Dougal Mackey, who co-led the study with Professor Geraint Lewis from the University of Sydney, said the international research team also found very faint traces of more small galaxies that Andromeda gobbled up even earlier, perhaps as far back as 10 billion years when it was first forming.

“The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way’s ultimate fate,” said Dr Mackey from the ANU Research School of Astronomy and Astrophysics.

“Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalised many more galaxies, possibly larger ones.”

The signs of ancient feasting are written in the stars orbiting Andromeda, with the team studying dense groups of stars, known as globular clusters, to reveal the ancient mealtimes.

“By tracing the faint remains of these smaller galaxies with embedded star clusters, we’ve been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times,” Dr. Mackey said.

The discovery presents several new mysteries, with the two bouts of galactic feeding coming from completely different directions.

“This is very weird and suggests that the extragalactic meals are fed from what’s known as the ‘cosmic web’ of matter that threads the universe,” said Professor Lewis from the Sydney Institute for Astronomy and University of Sydney School of Physics.

“More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre ‘plane of satellites’, an unexpected alignment of dwarf galaxies orbiting Andromeda.”

Dr. Mackey and Professor Lewis were part of a team that previously discovered such planes were fragile and rapidly destroyed by Andromeda’s gravity within a few billion years.

“This deepens the mystery as the plane must be young, but it appears to be aligned with the ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation,” Professor Lewis said.

“We’re going to have to think quite hard to unravel what this is telling us.”

Dr. Mackey said studying Andromeda also informed understanding about the way our galaxy has grown and evolved over many billions of years.

“One of our main motivations in studying astronomy is to understand our place in the Universe. A way of learning about our galaxy is to study others that are similar to it and try to understand how these systems formed and evolved.

“Sometimes this can actually be easier than looking at the Milky Way because we live inside it and that can make certain types of observations quite difficult.”

The study, published in Nature, analyzed data from the Pan-Andromeda Archaeological Survey, known as PAndAS.

“We are cosmic archaeologists, except we are digging through the fossils of long-dead galaxies rather than human history,” said Professor Lewis, who is a leading member of the survey.

The team involved institutions from Australia, New Zealand, the United Kingdom, Netherlands, Canada, France, and Germany, and used the Canada-France Hawaii Telescope, the Gemini North Telescope, the Keck Telescope, the NOAO Mayall 4m telescope, and the William Herschel Telescope.


Comments

The expansion of the Universe, as measurements carried out by astronomer Edwin Hubble in the 1920s show, mean that galaxies are rushing away from us at a rate, recently measured by today’s cosmologists, to be 74 kilometres per second per megaparsec (where one megaparsec equals around 3.26 million light years).

While it is easy to envision all galaxies moving away from each other, the evidence of smash-ups between these gigantic structures litter the Universe. This means that galaxies are both moving away and crashing into one another – this happens much more often than you think.

So often, in fact, that our galactic neighbour, Andromeda is moving towards the Milky Way Galaxy at around 250,000 miles per hour – a speed that would get you to the Moon in about an hour.

Why this is so is all thanks to the gravity of the dark matter surrounding the pair, knitting them together so tightly, that they resist the expansion of the Universe and are instead, drawn together with Andromeda falling towards us. As you may have read in our feature on the Andromeda Galaxy in issue 6 of All About Space, we are unlikely to see the spectacular collision as our Sun evolves and extinguishes life on our planet’s surface.

However, when the inevitable does happen, and the two coalesce, they will create a single elliptical galaxy with the merger triggering a great burst of star formation and the supermassive black holes that sit at the hearts of both galaxies will combine. While stars in both the Milky Way and Andromeda are unlikely to collide due to their great distances, the gravitational disturbance could cause what is left of our Solar System to change its position – tossing it from its current position in the Orion spur and further from the Milky Way’s core.

The galaxy merging does not end there either Andromeda’s companion, the Triangulum Galaxy – which is also attached by dark matter to the pair – will join the collision, taking another two billion years to merge .with “Milkomeda” completely.


Andromeda and the Milky Way: A Merger of Galactic Proportions

Visit Andromeda before it visits you. The Milky Way galaxy, home to the Sun and its few billion close friends, has a sibling, a slightly larger big brother galaxy known as the Andromeda nebula, M31, in astronomical parlance. The pair, the Milky Way and Andromeda, are the dominant members of a small family of galaxies known as the Local Group. Whereas the universe is expanding and galaxies are generally getting farther and farther away from each other with time, the galaxies in the Local Group are bound together by family ties in the form of their mutual gravity. And so our neighbors aren’t going anywhere. And there is the problem. Andromeda and the Milky Way are actually heading toward each other in the dose-do that constitutes life in a galaxy cluster. Recent measurements with the Hubble Space Telescope have confirmed that they will hit head on in about 2 billion years. Since galaxies, like atoms, are mostly empty space, they will pass through each other like ghosts, but gravity will disrupt the stars and strew them all over space in gigantic spectacular streamers. Eventually they merge into a single giant galaxy. The bad news is that we will be dead. The Earth will have been boiled and sterilized eons earlier as the Sun swells and dies. The good news is that the collision will be a fiesta of new stars forming as that disruptive gravity collapses clouds of gas and dust and they make new stars. New worlds, another chance. Maybe.

The Andromeda nebula, which rarely feels the pull of the social media orbit, had a moment in the spotlight on Wednesday. Astronomers operating NASA’s Swift satellite spied what looked like a giant burst of radiation from Andromeda, the nearest big galaxy to our own Milky Way, about 2.5 million light-years from here. They tentatively diagnosed it as the collision of two neutron stars, the dense remnants of dead stars. Such collisions are among the most violent known conflagrations in the universe, but they rarely occur so close to our own neck of the cosmic woods.

It turned out to be a false alarm, but for a few hours the Twitterverse was riveted on Andromeda. Which is not a bad thing. The Andromeda galaxy, known in astronomical parlance as M31, holds a special place in our own future.

The Milky Way and Andromeda are the dominant members of a small family of galaxies known as the Local Group. Whereas the universe is expanding and galaxies are generally getting farther and farther away from one another with time, the galaxies in the Local Group are bound together by family ties in the form of their mutual gravity. Our relatives aren’t going anywhere.

And there is the problem. Andromeda and the Milky Way are actually heading toward each other in the do-si-do that constitutes life in a galaxy cluster. Recent measurements with the Hubble Space Telescope have confirmed that they will hit head on in about two billion years. Since galaxies, like atoms, are mostly empty space, they will pass through each other like ghosts, but gravity will disrupt the stars and strew them across space in gigantic spectacular streamers. Eventually they will merge into a single giant galaxy.

The bad news is that we will be dead. Earth will have been boiled and sterilized eons earlier as the sun brightens. The good news is that the collision will be a fiesta of new stars forming as that disruptive gravity collapses and then condenses clouds of gas and dust. New worlds, another chance. Maybe.


Milky Way doomed to head-on crash with Andromeda

(Space.com) Four billion years from now, the Milky Way galaxy as we know it will cease to exist.

Our Milky Way is bound for a head-on collision with the similar-sized Andromeda galaxy, researchers announced today (May 31). Over time, the huge galactic smashup will create an entirely new hybrid galaxy, one likely bearing an elliptical shape rather than the Milky Way's trademark spiral-armed disk.

"We do know of other galaxies in the local universe around us that are in the process of colliding and merging," Roeland van der Marel, of the Space Telescope Science Institute in Baltimore, told reporters today. "However, what makes the future merger of the Andromeda galaxy and the Milky Way so special is that it will happen to us."

Astronomers have long known that the Milky Way and Andromeda, which is also known as M31, are barrelling toward one another at a speed of about 250,000 mph (400,000 kph). They have also long suspected that the two galaxies may slam into each other billions of years down the road. [Milky Way Slams Into Andromeda (Artist Images)]

However, such discussions of the future galactic crash have always remained somewhat speculative, because no one had managed to measure Andromeda's sideways motion -- a key component of that galaxy's path through space.

But that's no longer the case.

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Van der Marel and his colleagues used NASA's Hubble space telescope to repeatedly observe select regions of Andromeda over a seven-year period. They were able to measure the galaxy's sideways (or tangential) motion, and they found that Andromeda and the Milky Way are indeed bound for a direct hit.

"The Andromeda galaxy is heading straight in our direction," van der Marel said. "The galaxies will collide, and they will merge together to form one new galaxy." He and his colleagues also created a video simulation of the Milky Way crash into Andromeda.

That merger, van der Marel added, begins in 4 billion years and will be complete by about 6 billion years from now.

A future cosmic crash

Such a dramatic event has never occurred in the long history of our Milky Way, which likely began taking shape about 13.5 billion years ago.

"The Milky Way has had, probably, quite a lot of small, minor mergers," said Rosemary Wyse of Johns Hopkins University in Baltimore, who was not affiliated with the new study. "But this major merger will be unprecedented."

The merger poses no real danger of destroying Earth or our solar system, researchers said. The stretches of empty space separating the stars in the two galaxies will remain vast, making any collisions or serious perturbations unlikely.

However, our solar system will likely get booted out to a different position in the new galaxy, which some astronomers have dubbed the "Milkomeda galaxy." Simulations show that we'll probably occupy a spot much farther from the galactic core than we do today, researchers said.

A new night sky

And the collision will change our night sky dramatically. If any humans are still around 3.75 billion years from now, they'll see Andromeda fill their field of view as it sidles up next to our own Milky Way. For the next few billion years after that, stargazers will be spellbound by the merger, which will trigger intense bouts of star formation.

Finally, by about 7 billion years from now, the bright core of the elliptical Milkomeda galaxy will dominate the night sky, researchers said. (The odds of viewing this sight, at least from Earth, are pretty slim, since the sun is predicted to bloat into a huge red giant 5 or 6 billion years from now.)

In its 22-year history, Hubble has revolutionized the way humanity views the cosmos. The new finding is another step in that process, researchers said.

"What's really exciting about the current measurements is, it's not about historical astronomy it's not about looking back in time, understanding the expansion of the universe," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate and a former astronaut who flew on three space shuttle missions that repaired Hubble .

"It's looking forward in time, which is another very human story," Grunsfeld added. "We like to know about our past -- where did we come from? We very much like to know where we're going."