Is Dark Energy Pushing Our Galaxy Somewhere?

If dark energy is pushing our Galaxy in a known and straight line, then where is it pushing us to? What kind of force is driving the dark energy and where is it taking us to? Or is there a force that is drawing dark energy to it, and we are just trapped within it?
Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Originally posted on Forbes!

Dark energy isn't pushing our Galaxy in a specific direction; it's responsible for the expansion of the space between all objects in space which are not tied to each other by gravity.

So, for instance, dark energy doesn't influence the distance between the Sun and the Earth, because our gravitational ties are much stronger than the gentle stretching that space is doing. Similarly, our Sun isn't being moved relative to the center of our Galaxy, because the force of gravity binding us to the Galaxy is much stronger than what dark energy can exert.

Dark energy can't even shear apart the gravitational ties which attach our Milky Way to the Andromeda Galaxy and the numerous, tiny dwarf galaxies which hover around our Galaxy in their own orbits. The distances here are enormous; Andromeda is 2.5 million light years away. Light arriving to us from Andromeda now will have left that galaxy when our planet Earth had only just seen its first humans.

Dark energy is a force to reckon with only for galaxies much more distant from us, where the gravitational force between our Milky Way Galaxy and that faraway galaxy plays no role. It's often phrased as a gravitational counter-force, but that's only partially correct. It is true that dark energy seems to have a repellent influence on the space surrounding it, but unlike gravity, which is strongest around concentrations of mass in the Universe, dark energy seems to be evenly spread throughout the universe, with not a care for the presence of a galaxy, planet, or supercluster . It's this evenhandedness of dark energy that means that gravity can overpower it in densely populated regions of the Universe.

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

Dark energy is not a directional force - there's no bulk motion to the left, right, or an arbitrarily defined up that this expansion leans towards. So there's no point to which the universe is being drawn, and equally there's no origin point from which the expansion is unspooling. Any given point in space is simply, and very gradually, becoming more distant from most other points in the universe. It's not that our Galaxy is being pushed around- dark energy is instead ballooning out the space within which our Galaxy sits.

Where does dark energy come from? So far that's a mystery; we can measure its influence to a great degree of confidence, but if we knew the exact nature of why it behaves the way we observe it to, we'd probably rename it something less vague than 'dark energy'.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

In the future, will we need more time to travel between galaxies?

Each galaxy is expanding away from the others. As a result, would that mean we require more time in the future to travel to other galaxies as the space in between increases as well?
A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across) Image credit:  Andrew Pontzen/Fabio Governato

A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across) Image credit: Andrew Pontzen/Fabio Governato

Originally posted at Forbes!

It depends on how far you want to go! Generally, when an astronomer is trying to explain the idea of space expanding, they might use the idea of the space between two galaxies expanding. This is a good illustration; it communicates the idea that it’s not that the galaxies are in motion themselves, but that the space they’re embedded within is growing, so the distance between the two of them wind up growing with time.

Unfortunately, this explanation can have a bit of an unintended side effect as a metaphor- if I’m not careful, I can wind up simplifying the universe too much, so that it seems like this happens equally to all galaxies, with galaxies spaced out roughly equally from each other across the universe. But if we actually look out into the night sky, we can see that ours is not a universe of equal spacing. Ours is a universe that looks rather spidery, with filaments of the cosmic web stretching across the sky. The image at the top shows a simulation of the distribution of galaxies in our universe. There are grand areas of nothing, and clusters of thousands of galaxies, connected to other clusters with faint tendrils, each made up of glowing galaxies.

And yet, the universe is expanding, and the space between objects is increasing, due to the currently inexplicable influence of Dark Energy. Should this affect, say, the distance between the Milky Way and Andromeda?

In principle it might – but it’s important to remember two facets to the universe’s expansion. One – this expanding force is very, very small. It happens that we have an awful lot of space, so over the whole universe it’s quite significant. The second thing is that on reasonably small scales (which in this case, means objects which are the size of galaxies and smaller), gravity is much stronger.

Gravity is a bit of a juggernaut in the astrophysical world, and if two objects are gravitationally bound to each other (meaning that their relative speeds are too slow to let them escape the pull of the gravity of the other), cosmic expansion isn’t going to be able to do much about it. The expansion of the universe would have to be frighteningly enormous (much larger than we observe it to be) to pull apart two galaxies which are bound to each other by gravity. The Milky Way and Andromeda are bound, and as such, are due to collide sometime in the next 3.5 – 4.5 billion years. So if you want to go to Andromeda at any point in the future (before it collides with us), cosmic expansion won’t play a role in the time it would take to travel there. It’s not just Andromeda that we’re bound to – we have a whole cluster of galaxies that our galaxy is part of, and all of these are bound to each other gravitationally, though much more loosely than the Milky Way is to Andromeda.

The location of the Milky Way with respect to the other galaxies within our Local Group. Image credit  Andrew Z. Colvin, CC 3.0 BY A-SA 3.0

The location of the Milky Way with respect to the other galaxies within our Local Group. Image credit Andrew Z. Colvin, CC 3.0 BY A-SA 3.0

But if you wanted to go beyond our cluster of galaxies, and travel intergalactic space to reach a totally separate cluster, one entirely unrelated to our galaxy – then, yes, you would need more time to travel to those galaxies in the future than you would if you left today.

Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.

Sign up for the mailing list for updates straight to your inbox!

BONUS ANNOUNCEMENT!

Help me do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Astroquizzical readers. By participating, you’ll be helping me improve Astroquizzical and you’ll be contributing to SCIENCE (on blog readership).

You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

Go science it up, space-curious Astroquizzical readers!

But if you wanted to go beyond our cluster of galaxies, and travel intergalactic space to reach a totally separate cluster, one entirely unrelated to our galaxy – then, yes, you would need more time to travel to those galaxies in the future than you would if you left today.



BONUS ANNOUNCEMENT!

Help me do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Astroquizzical readers. By participating, you’ll be helping me improve Astroquizzical and you’ll be contributing to SCIENCE (on blog readership).

You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

Go science it up, space-curious Astroquizzical readers!