Transcript: What Lies Between Galaxies? Ejected Stars, Rogue Planets and Exotic Matter

We often view intergalactic space as a no man's land of empty space-time. And, it mostly is, about the most you'll find at most points within it are diffuse hydrogen atoms passing by. But there are some objects wandering the lonely reaches of intergalactic space, including stars and planets. And, it's just possible that there may even be somewhere in this universe an isolated civilization living amongst the black expanses.

One kind of object you might find wandering the space between thegalaxies are ejected stars, often called rogue stars. These are stars that presumably formed inside galaxies and then were ejected out. There are two mechanisms by which this is thought to happen. The first is during the gravitational chaos that occurs when two galaxies collide and the second is when a multiple star system gets too close to a black hole. One member of such a system would get sucked into the black hole, whereas the other or others would be flung out into space. There it becomes something known as a hypervelocity star.

Hypervelocity stars are just that, stars moving at very fast speed, and that can be enough to escape the gravity of their parent galaxy. But what's staggering about rogue stars is how many of them there apparently are. In 2010 and 12 an experiment called the Cosmic Infrared Background Experiment was launched using sounding rockets. The experiment found a strange glow coming from intergalactic space that could not originate from other galaxies.

The best explanation was rogue stars. But the sheer amount of light that was detected suggests that as much as half of all stars in the universe are wandering in intergalactic space. This is interesting because there is a mystery in particle physics called the "missing baryon problem".

Baryons are the particles that make up ordinary matter, a general term for protons, neutrons, etc. Most models of the early universe suggest that there should be way more baryons than there appear to be. But, if half of all stars are wandering intergalactic space, then that could go a long way in helping to account for the missing baryons.

But it likely wouldn't just be stars wandering intergalactic space. They might well take their planets with them ... and any life that might living on those planets. Passing so near a black hole isn't going to be good for life, but if it arises after the ejection then perhaps intergalactic space is teeming with life. Perhaps even civilizations exist out there completely unassociated with galaxies.

There is actually one factor that might favor such life. Most galactic stars reside in high radiation environments hostile to life, such as near a galactic core or in a star cluster. The close proximity to other stars in this case is bad for life, planets near the galactic core would be repeatedly sterilized by close supernovas. But the further you get from the core, out into the spiral arms of the galaxy and beyond, the potential for life grows.

But, there would also be rogue planets traveling without a star in intergalactic space. Similar to stars, these planets would be thrown clear of their parent galaxy through gravitational encounters. We may never see one of these, it would be incredibly hard to spot such a thing in deep space, but they likely exist.

And there are even models where these kinds of worlds can harbor liquid water and life if the planet has a way of keeping warm, such as nuclear decay in the core. This might create an ice shell world, similar to Europa or Enceladus. Or, if you add a thick hydrogen atmosphere, you could have surface liquid water and who knows what else.

One last kind of object that may lie in between the galaxies is very different from stars and planets. It's a hypothetical form of exotic matter that would exhibit negative mass. We're still uncertain whether this kind of matter exists in nature, or for that matter if we could somehow synthesize it. But this form of matter is thought to be possible only in that it's mathematically sound and does not violate the laws of conservation of energy or momentum, but it may violate relativity.

That it could violate relativity in a kind of loophole features prominently when you hear technological theorists talk about creating artificial wormholes, traversing black holes and building alcubierre drives to go faster than light. Most models of these potential future technologies all require this kind of exotic matter to exist.

Whether we can make this stuff is likely not going to be cleared up any time soon. We don't have a complete enough view of gravity, all the current theories fall short and we really need a unified theory of everything essentially. That also happens to be one of the greatest mysteries in modern physics, something even Einstein couldn't figure out. But we need it to answer the questions surrounding negative mass. But, if such a material could exist. What would it be like?

It would be extremely strange indeed. A normal object might weigh 5 kilograms on earth. But an object with negative mass would weigh negative 5 kilograms. Such an object would be expected to be repelled by gravity, in other words it would have the property of anti-gravity, and would fall upwards. Another odd expected effect of exotic negative mass matter is that when you push on it, it would push back. Moving furniture made of this material would be beyond difficult.

If there was some mechanism for this material to somehow have been created in the big bang, which is a huge if, it might well have been repelled by the galaxies completely and potentially lurks in deep space at points between them.

Thanks for listening! I am futurist and science fiction author John Michael Godier and if you would like to help support the channel, check out my Patreon page, link in the description below or check out my books at your favorite online book retailer and subscribe to my channel for regular, in-depth explorations into the interesting, weird and unknown aspects of this amazing universe in which we live.