Beyond The Cosmic Horizon
Jessia Adachi
Where does the Universe end? Or, to put it differently, does the Universe have an edge? When cosmologists say that the Universe is expanding, people tend to think of an exploding bomb. They see galaxies as shrapnel, flying off in all directions. Even if intuitive, this image is dead wrong.
The cosmic expansion is an expansion of space itself. Since Einstein's theory of general relativity, space has been endowed with a plasticity that allows it to expand, shrink or fold like a rubber balloon in response to the presence of matter (and energy).
Galaxies are like islands in an ocean (in three dimensions, though), distance markers that are carried along by the expansion. Think of floating logs on a river, for example. If the galaxies have any additional motion, say, when two are near each other and attract gravitationally, this motion is superimposed on the inexorable cosmic drag. (It's actually called "peculiar motion.")
One of the consequences of the cosmic expansion is that the Universe has no center. (An assumption here, which has been confirmed so far by observations, is that the Universe, which includes all of observable space and matter in it, is homogeneous, that is, the same everywhere when averaged over large enough distances.)
Imagine that you, from your galaxy, observe other galaxies around you. Due to the expansion, most of them are moving away from you. You'd then conclude that you must be the center, since every other galaxy is receding from you. However, an observer in another galaxy will see the same thing: every one else receding from her. The same with each and every galaxy. In the Universe, space is the ultimate democracy: all points are equally important.
But if this is true, how to explain the smallness of space near the Big Bang? If the Universe is expanding, in the past distances were shorter. Astronomers can measure the recession speed of galaxies and, from them, project when they would have been "on top" of one another in a tiny volume. This moment marks the beginning of our cosmic history. According to modern measurements, this happened some 13.7 billion years ago, about three times longer than the age of the Earth. Even here, as long as we can talk about space, all points on it are equivalent.
Only when we reach the "singularity" do things break down: however, the notion that space shrinks to nothing at time zero is an extrapolation based on classical ideas and is surely wrong: new physics kicks in when the Universe is very small.
When we consider the finite cosmic history and the speed of light together, we arrive at a key concept, that of the cosmic horizon: since the speed of light delimits the speed with which we can receive information, in a Universe with a finite age we can only receive information from objects situated at the maximum distance that light could have covered within this time. Somewhat like the horizon at the beach, that delimits how far we can see.
But the ocean doesn't end at the horizon: there is more water beyond. What about the Universe? It also keeps on going. Probably. We can't be completely sure since we can't get information from beyond the horizon. And how far is that? If the Universe weren't expanding, the distance to the horizon would be 13.7 billion light-years. But since space gets stretched with the expansion, light waves get lift and we can see further than that: the cosmic horizon is roughly at 42 billion light-years away.
Beyond the horizon, the Universe could keep on going, if space is indeed infinite in all directions. It could also be closed on itself, like the surface of a balloon (but in three dimensions, not something easy to visualize), or be really weird beyond what we can see. The existence of a cosmic horizon implies a fundamental limitation to how much we can know: we are partially, if not totally, blind to what lies beyond. There could even be a multiverse out there.
is assistant professor of physics at North Carolina State University, where she is also a member of the Leadership in Public Science Cluster. She is the author of The End of Everything (Astrophysically Speaking) (2020).
The distance to our cosmic horizon is not, as you might expect, 13.8 billion light-years. As we discussed above, distances are weird in an expanding universe. Something that was 13.8 billion light-years away when its light started the journey toward us is much farther away now. If you factor all that in, that glowing plasma we see at the very edge of the observable universe is actually somewhere around 45 billion light-years away now.
The patterns in the cosmic microwave background light, the distribution of galaxies, and even experiments testing gravity and the behaviour of particle physics are giving us insight into the fundamental structure of the Universe, and into its evolution in its earliest moments. We are getting closer and closer to being able to tell our whole cosmic story. We can already see, directly, the fire in which our universe was forged, the moments just after its beginning. With the clues we are gathering now, we might, someday, follow the story all the way to its end.
This Essay was made possible through the support of a grant to Aeon from the John Templeton Foundation. The opinions expressed in this publication are those of the author and do not necessarily reflect the views of the Foundation. Funders to Aeon Magazine are not involved in editorial decision-making.
There are three widely used definitions of cosmological horizons, which are limits imposed by cosmology to how far away we can see. The purpose of this post is to try and explain in a non-technical way these different definitions. I hope I succeed!
The particle horizon is the theoretical maximum proper distance we can see to at the current time. It is a spherical shell approximately 46.5 billion light years in radius around the Earth. When we look at distant objects we are looking back in time and light from an object at the particle horizon will have been emitted at the beginning of the Universe and will have been travelling towards us for the entire age of the Universe.
All the objects we observe today lie inside the particle horizon, which forms the boundary of the observable universe. If an object lies beyond the particle horizon, then the Universe is not old enough for its light to have had enough time to reach us.
(2) Because the particle horizon is the proper distance of the furthest object we can see, due to the expansion of the Universe, as the Universe ages the proper distance between two distant objects increases.
In reality, we cannot see all the way to the particle horizon. As readers of my earlier post will know, the early Universe was far too hot for atoms to exist. It contained a plasma of positively charged hydrogen and helium ions and negatively charged electrons. Electromagnetic radiation, of which light is an example, cannot pass through plasma. The oldest radiation we can detect is the cosmic microwave background (CMB) which was emitted when the Universe was only 400 000 years old, at which time it had cooled sufficiently for individual atoms to exist. The CMB radiation we observe today has been travelling towards us since this time and was emitted from a spherical shell of points, which lie at a proper distance of approximately 46 billion light years from Earth.
However the Hubble parameter is changing over time, so we need to consider a further type of horizon, the event horizon. This is the largest proper distance from us from which light emitted now will reach us at some distance time in the far future.
The graph below shows how the event horizon changes over time. In the current model of the Universe the event horizon will gradually increase with time but at a slower and slower rate reaching a maximum value of around 18 billion light years.
Distant galaxies are receding from us superluminally. This means we will never be able to see their light emitted at the current time. However, they are at rest locally and motion in their own local inertial frames remains is well described by special relativity. For more details see Davis and Lineweaver (2003).
Firstly, as discussed in the post
The particle horizon is the theoretical maximum proper distance we can see to at the current time. It is a spherical shell approximately 46.5 billion light years in radius around the Earth. When we look at distant objects we are looking back in time and light from an object at the particle horizon will have been emitted at the beginning of the Universe and will have been travelling towards us for the entire age of the Universe.
The age of the Universe depends on the various parameters in our model of the Universe. Essentially what we do is measure its current density, composition and rate of expansion and work backwards to time when it had an extremely high density. The following link may prove useful
I did not read the entire content, but how did the particle horizon reached 46.5 billion light years in radius IF the age of universe is only at 13.8 billion years since big bang? Should it not be only atmost 13.8 billion light years(the farthest the light would have traveled since big bang)?
Hi Rodel, that is due to cosmic expansion of the fabric of space, so while the cumulative separation could be at well in excess of light speed, the objects themselves are just in their normal orbital peculiarities. The classic analogy example is leavening raisin dough. This holds true under both SCM-LCDM consensus and the competing SPIRAL cosmological redshift hypothesis and model.
Nice presentation Steve.
Please advise approximate LY distance of the original departure point radius, of the CMB radiation we see here and now, that has travelled 13B rounded LY to get here.
was it at that distance x at the end of 400k years after the start of the big bang? or the end of cosmic inflation?
also
what is the nearest known departure point of any light we see here and now that has ever been subjected to any comic expansion?
by definition any light arriving here and now that has any degree of cosmological redshift (CR) has been subjected to cosmic expansion, is that LY distance closer than to the nearest stellar object whose light has any CR?
distance where it is now and how much closer it was when the light departed it, please.
TY,
rm