Once Around Dark Flow

Added: 2026-06-04

[00:00:02]Once around dark flow.

[00:00:06]So dark flow is another name that goes along with the two names that we probably all have heard associated with other dark stuff. Dark matter, the extra unseen mass whose gravity causes galaxies to spin faster than we expect based on all of the material that we can see and detect. with all of our different telescopes at different wavelengths. Everything from gamma radiation through visible light down to radio waves. We can add it all up and there's just not enough of it to explain how a galaxy can h hold together when rotating as fast as it does or indeed hold groups of galaxies together in large galaxy clusters like the coma cluster shown at the bottom there where Fritz Vicki first came up with the uh problem that there seemed to be 450 times too much mass to explain how the cluster held itself together under gravity. And then we have dark energy, the name given to the effect whereby it appears that the universe is accelerating in its expansion rather than slowing down and gravity should be gradually slowing it down, causing the initial expansion rate to get lower and lower, perhaps never actually reversing it. But the observations made in the late 1990s and the theories that were associated with dark energy seem to suggest some sort of mystery negative pressure pushing faster and faster from empty space itself generating this so that the more space there is the more pressure there is the faster everything flies apart. Now, that's been called into question recently and is the subject of another talk of mine. In fact, so is dark matter. And we're [snorts] not here to talk really about either of those.

[00:02:10]What we are looking at is the fact that whole clusters of galaxies are moving independently of the expansion rate of the universe. They all do that. They all have their peculiar motion as it's known, their individual motion relative to the wider large scale average. But in one particular corner of the visible universe, a whole group of galaxies in clusters, in fact, several clusters making superclusters are all moving towards one region of the sky. And this is rather difficult to explain.

[00:02:52]When we look way back in time to the period just 380,000 years after the Big Bang, we see the afterglow, the cosmic microwave background radiation, the CMBB, as an almost uniform temperature across the sky. 2.7° above absolute zero is the measurement we get for it at the moment. Plus or minus about one part in 100,000. And those tiny fluctuations are what's shown in the false color map of the sky at the top there with red being intense uh slight increases and blue being the cold spots that are just 100,000th of a degree too cold compared to the average. So very tiny. But this radiation is the leftover heat that was able to finally flow through the universe as radiation unimpeded once the overall temperature of the gas in the universe dropped to the point where full atoms could form and they became transparent around about 4,000 degrees Kelvin. Now, expansion of the universe has stretched that radiation from 4,000 Kelvin to 2.7 Kelvin by a factor of well over a thousand now. And that has made the wavelengths longer, moving the light from a sort of orange glow in the visible part of the spectrum, the color of the surface of an orange small star perhaps down to this long wavelength in the microwave region and is detectable only with our radio telescopes.

[00:04:46]One of which was the W map satellite.

[00:04:49]And in 2008, there was an analysis of the data from three years of W map surveying of the temperature of the microwave background across the sky. And the observations were looking out to around 1 billion light years away from the solar system, away from the Milky Way. And there was a hint that there was a movement shown in that purple region there that's been superimposed on the image to show this region of the sky that seemed to be where material was moving away from us faster than typical perhaps a Doppler shift as the uh temperature of the radiation was altered by the apparent movement away from us giving it extra red shift. And the cluster of galaxies in that region is shown in the inset there. So that was 2008. Well, it was followed up in 2010 by Kashinski in the Godart Space Flight Center.

[00:05:57]Now looking at a thousand galaxy clusters, a much larger sample, and going over double the distance to 2.5 billion light years. And you can see that the mapping has changed depending on where you look. So we have the furthest away galaxies with the deepest red color uh then orange, yellow, green, and blue moving from at most 2.5 billion light years away to the much closer region around 1 billion lighty years away. And you can see that the center or point of that zone has shifted as you go further further back in time. Now the way that this analysis has been done uses an effect which I want to explain to you and it's the SZ effect. The sonif zelvich effect. Uh this is not widely talked about but it's an effect whereby the cosmic microwave background radiation and it the spectrum of it is altered as the radiation passes through material on its way to us and into our radio telescopes. And the picture at the bottom there is the big Amy experiment, the Royal Array here in Cambridge uh at the Mard Radio Astronomy Observatory just southwest of the city there. Um and I've visited there a number of times going back again in June actually taking a a party of interested people on a tour.

[00:07:36]And the big Amy is one of the experiments that is still running and it's in conjunction with little Amy uh which the two together are able to look at this SZ effect in two different ways and the data combined from the two experiments is really what we're talking about here.

[00:07:59]The first is to look at the thermal SZ effect, which is where the overall temperature of the cosmic microwave background is altered by the fact that the photons get a kick from thermal electrons in clouds of ionized material. As the photons go through, they get clouted by electrons and they pick up a little bit of an extra kick, little bit of extra energy and so that can be determined. And then we have the kinematic as said effect which is where the material is moving and essentially you get a Doppler shift effect of the movement of the electrons relative to the uh cosmic microwave background photons giving a a different imprint on the spectrum. And it's by combining the two experiments that we're able to measure this uh effect and determine that there are regions of ionized material clouds of protons and electrons from ionized hydrogen associated with these galaxy clusters that are moving away from us because of the Doppler shift.

[00:09:20]Now, the Plank satellite got in on the act with a much higher resolution version of the map of the cosmic microwave background radiation back in 2013. it seems like just yesterday and seemed to show no evidence of this effect at all in the initial analysis.

[00:09:41]But two years later, 2015, um scientists were able to see that there is some evidence for this peculiar motion going on out there in the very distant parts of the universe. Uh, but I wouldn't say this is settled yet. But what I want to talk to you about is the idea that it might be something that's a real detection. But as with all of these things, we're pushing the boundary of what is known.

[00:10:14]So what did we think we have found?

[00:10:17]Well, somewhere in this region marked in the huge red ellipse there spanning the constellations of Centurus in the middle, Hydra at the top and Norma at the bottom. There is this area which seems to be suffering this peculiar motion away from us in this very very distant region 2.5 billion light years away across the the universe. Now this turns out to be the direction of something called the greater tractor.

[00:10:53]And I've done a video about the greater tractor which you might want to go and search for. And this is in fact in the constellation of Norma probably a huge cluster of galaxies, the Norma cluster.

[00:11:10]And it seems to be the source of immense gravity pulling on ourselves, the Virgo cluster that we're part of and a lot of other material all moving in that direction in a in a region that we pretty much understand. We understand how fast we're moving towards it and can estimate the mass. And there's no particular problem with this except for the fact that 250 million years is relatively nearby. Sorry, 250 million light years is relatively nearby.

[00:11:44]Not the 2.5 billion that we're talking about. That must be 10 times further away in this general direction. So, we're not actually looking at the greater attractor being part of this pull. In fact, it would be pulling the material back towards it in the other direction. So, it's not the great attractor. This is something much more distant, possibly even so far away that we can't see it. And I don't mean that we haven't seen it, but that I mean that it might be beyond our cosmic horizon.

[00:12:22]some sort of huge accumulation of matter whose gravity is affecting the very distant universe pulling it towards something that we cannot see because the light from that large mass whatever it is has not yet had time to reach us yet.

[00:12:45]So this points towards the idea of our cosmic horizon.

[00:12:53]We can only see as far as the light travel time since the big bang. The further we look, the further back in time we are looking all the way back to those early times with the cosmic microwave background actually being the limit as far as visible light is concerned. Um, and that's very very close indeed all the way back to the Big Bang. It's just 380,000 years after the beginning. Um, which is some 13.7 billion years ago.

[00:13:28]So, we can't see any further than that. Of course, as the universe gets older, we would expect that we would be able to see light that has traveled further. And so that cosmic horizon you might expect to increase in size. Uh the problem is that as space is expanding if you look far enough away there is a point where expansion is carrying material away from us faster than the speed of light. It's not moving through space. It's the space growing faster than the speed of light. Meaning that the light will never get here. So one way or another there is a cosmic horizon and that cosmic horizon is personal to all of us. I have a diagram here showing the idea with the earth in the middle.

[00:14:22]Um there is no middle but we might as well put ourselves somewhere and showing that either side of us we have objects A and B which are right on the limit of the light travel time. So that we have just received the first photons that set off from A and from B in opposite directions and we can just detect their presence right at the limit of our cosmic horizon.

[00:14:54]Likewise, photons from Earth will just be arriving at A and they'll be just arriving at B having traveled for 13.78 billion years. Now the problem is that whilst we can see both A and B and they can each see us, they can't see each other because light from A has only reached halfway to B under this model.

[00:15:19]And so each of them sees a sphere around them which is their cosmic light horizon. And uh we all have different ones. Yours is different to mine. mine is different to people who are on the uh Andromeda galaxy and so forth. And so maybe there is some large mass creating this dark flow as we call it pulling perhaps object A towards something that exists within A's cosmic light horizon but does not yet exist within ours. And as I said because of the expansion of the universe carrying very distant things away from us that might never become true. It might get carried away faster than the light is able to cross the space and so the photons might never arrive at earth. Um and the these are two different horizons in fact.

[00:16:22]But another explanation is that maybe this is outside influence.

[00:16:28]The theory of inflation that describes the very very earliest instance of time between the big bang and the tiniest millionth of a second afterwards talks about a universe of cosmic foam expanding exponentially way faster than the speed of light. blowing it up to a huge size and explaining why the uh cosmic microwave background is so uniform because it used to all be in a tiny space and all in causal contact before inflation ripped it apart into a truly vast size.

[00:17:15]And it talks to the fact that these cosmic bubbles could exist where different periods of inflation may have kicked off in different regions of an overall enormous inflationary universe.

[00:17:29]And that perhaps our big bang is just one of many.

[00:17:34]and that maybe that the dark flow is in fact the influence of something that is outside of our inflated bubble and wasn't ever part of our big bang at all.

[00:17:51]And so studying this and understanding if this is a real effect, if there are other cases of it elsewhere, pointing in different directions, mapping the cosmos to see if we can see these objects that are giving us hints of what lies beyond our reach could be a whole new branch of astronomy.

[00:18:15]So, thanks very much for listening to that little chat about the ideas of dark flow and what lies beyond the edge of the universe.