Explainer: What is the Great Attractor and its pull on the Milky Way?

May 31, 2016 by Lister Staveley-Smith, University Of Western Australia, The Conversation
An artist’s impression of the galaxies found in the ‘Zone of Avoidance’ behind our Milky Way. Credit: International Centre for Radio Astronomy Research

Around four decades ago, astronomers became aware that our galaxy, the Milky Way, was moving through space at a much faster rate than expected.

At 2.2-million kilometres an hour, the speed of the Milky Way through the cosmos is 2,500 times faster than a cruising airliner; 55 times more than the escape velocity from Earth; and a factor of two greater than even the galaxy's own escape velocity!

But where this motion comes from is a mystery.

The Big Bang theory of our origin tells us that every point in the universe should be flying apart from every other point. Nevertheless, on either side of us should be moving at similar recession velocities, which should result in no net motion in the Milky Way's frame of reference.

Net motion can arise from nearby clumps in the distribution of matter, like a massive cluster of galaxies. The additional gravitational attraction of such a galaxy cluster can slow down, and even reverse, the expansion of the universe in its immediate vicinity.

But no such cluster is obvious in the direction of the Milky Way's motion. There is an excess of galaxies in the general vicinity, and an excess of radiation visible in X-ray telescopes. But nothing that in any way seems large enough to explain the results.

So are we seeing an over-density of pure dark matter? Or is the current theory of the origin of mass and motion incorrect? Astronomer Alan Dressler, of the Carnegie Institution, used the former explanation, famously dubbing the missing concentration of matter the "Great Attractor."

But another explanation may lie in the fact that the inferred direction of the missing matter is not too far away from the direction of the Coalsack nebula, which lies deep within our own Milky Way.

The hidden attractor

Could our own Milky Way be moving through space like an edge-on spinning disc, obscuring the very source of a distant gravitational attractor? Could there be a super-massive cluster of galaxies (it would need to be the equivalent of 10,000 Andromeda galaxies) that is somehow being missed because its being obscured by the dense layer of dust associated with the thin disc of the Milky Way?

With that in mind, in the late 1990s, our team began using an innovative instrument on the iconic Parkes telescope, in New South Wales, simply known as the Parkes multibeam receiver. The unique sensitivity and field of view of this receiver allowed us to make progressively more sensitive radio surveys of the sky.

These surveys were made by tuning the receiver to what is known as the 21cm line of neutral hydrogen. Although a weak line, the sensitivity of the receiver was such that thousands of galaxies could be detected in "blind" surveys.

Moreover, at radio wavelengths, radiation passes straight through the dust layer in the Milky Way. The Milky Way essentially becomes invisible.

The HI Parkes All-Sky Survey (HIPASS) provided the first shallow survey of the whole southern sky. In fact, HIPASS was the first sensitive sky survey for extragalactic hydrogen ever made by any telescope. But nothing unexpected was found behind the Milky Way.

Other shallow surveys by our team targeted the Milky Way itself. Only a mild galaxy over-density was seen.

But it was recognised that deeper observations were needed. Theoretical models (particularly the so-called Lambda-cold-dark-matter model) only come under suspicion if nothing is found within a distance of 200 million light years.

Therefore, a long series of deeper observations of the local universe behind the disc and bulge of the Milky Way was conducted, again with the Parkes telescope.

From out of the data

These finished in the mid-2000s. Due to the extra difficulty of analysing radio data in the Milky Way (there is extra noise created by cosmic rays in our galaxy) and the dispersal of our team, it took until last year for all the data to be fully analysed and submitted for publication.

Within five degrees of the Milky Way's disc, we found altogether 883 galaxies plus a further 77 in the two bits of the northern Milky Way, visible from Parkes. Only a small number of these galaxies had a previous optical redshift and therefore distance estimate.

But when we looked at data from new infrared surveys, combined with data from our own brand-new deep infrared survey (infrared or heat radiation passes much more easily through dust), we were able to confirm stellar counterparts for almost 80% of the galaxies. The rest are too deeply embedded in the Milky Way to be confirmed with any existing optical or infrared telescope.

The discovery of so many previously hidden galaxies created quite a bit of excitement. But given that we didn't find the Great Attractor, why all the excitement?

It would be nice to think that it was because the mystery has deepened even more. We did find new galaxies, clusters of galaxies and new strands in the cosmic web. Just not enough to explain our motion, so there is still the mystery of what is the "Great Attractor" that has this pull on our Milky Way.

I think most of the excitement was generated by the simple act of unveiling the universe a little bit more, like the early explorers completing maps of the blank southern hemisphere.

Further exploration

So what comes next? It just so happens that Australian astronomers are in prime position to further explore structure and motions in the nearby universe. Radio surveys such as CAASTRO's 2MTF survey, which also uses the Parkes telescope to calculate galaxy distances, are already making new contributions.

Better still will be the WALLABY survey, for which I am co-principal investigator with Dr Baerbel Koribalski, to be executed with the new CSIRO Australian SKA Pathfinder (ASKAP) telescope, starting later this year.

This will allow us to make massive inroads into the detailed exploration of the radio universe, as will the Square Kilometre Array (SKA) itself. At optical wavelengths, the Australian Astronomical Observatory and the ANU are leading a new survey, called TAIPAN, which will target elliptical galaxies to explore more distant regions.

Theorists are also exploring whether the space-time metric we use to describe the universe may no longer be valid, and whether general relativity itself may need modification on large scales.

It's early days yet, and major shifts in the cosmological paradigm require incontrovertible evidence. Nevertheless, the mystery behind the Great Attractor is an enduring one and may not be fully understood for a few more years.

Explore further: Scientists discover hidden galaxies behind the Milky Way

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28 comments

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OdinsAcolyte
5 / 5 (1) May 31, 2016
Who knows? It could all be an invisible topography.

Great work fellows
OdinsAcolyte
5 / 5 (1) May 31, 2016
Who knows? It could all be an invisible topography.

Great work fellows
Otto_Szucks
1.4 / 5 (9) May 31, 2016
"Theorists are also exploring whether the space-time metric we use to describe the universe may no longer be valid, and whether general relativity itself may need modification on large scales."

Space-Time or *spacetime" as included in math formulas/equations has no meaning with regard to the TIME portion. The Theory needs overhauling due to this mistake by the master himself, Einstein. Time is NOT a dimension and never was. But it is taught by instructors as though it were an essential part of science.
How do you unlearn this bizarre component that is so illogical that one must wonder how it ever made it in there in the first place.
Gigel
5 / 5 (1) Jun 01, 2016
"At 2.2-million kilometres an hour, the speed of the Milky Way through the cosmos"

That's about 600 km/s
antialias_physorg
4 / 5 (4) Jun 01, 2016
Time is NOT a dimension and never was. But it is taught by instructors as though it were an essential part of science.

Ermm...how about: no? There's a reason why Einstein called it "spacetime" and not "space and time" - and the correct formalism is taught in every physics course, everywhere.

That time can be *approximated* in many fields as a separate dimension isn't an error (basically any field that doesn't work at relativistic speeds). It's like Newtonian gravity is an adequate approximation for many fields.
cantdrive85
1.8 / 5 (5) Jun 04, 2016
Once again observation falsifies the BB creation event and the psuedoscience GR. All that is required though is an ad hoc band-aid to sweep these findings under the cosmic rug.

And yes, any mention of "spacetime" relegates GR further into the obsolete dustbin of pseudoscience.
TheGhostofOtto1923
3 / 5 (2) Jun 04, 2016
Space-Time or *spacetime"
-or Space_Time or SPACETIME or spaace..tiiime... or tick-tock-travel-realm or ?
as included in math formulas/equations has no meaning with regard to the TIME portion. The Theory needs overhauling due to this mistake by the master himself
wait for it
Einstein
-oh I thought you were going to say god the holy savior of the universe. My mistake.
How do you unlearn this bizarre component that is so illogical
-This from the person who is so proud of saying things like
The cell had all its ducks in a row, so to speak, and God only had to use His Energy to start up the tiny "machine" so that it could become self-aware with EM coursing through its neurons, etc
-and of course, re martian heads
semi-transparent, not translucent. We also have independent, PROFESSIONAL verification as to the existence of the humanoids in the Mars pictures
That whole thread is a hoot BTW.
Reg Mundy
2 / 5 (4) Jun 04, 2016
Theorists are also exploring whether the space-time metric we use to describe the universe may no longer be valid

Its "time" to rethink the relationship between space(and all the matter in it) and time. As I've explained many "times", time is subjective and quantum, each quantum being defined by the positions of the universe's particles and chosen in sequence from the infinite variety of patterns by the laws of physics on a macro scale (which is why they don't apply on a micro scale). Only by restating physics in a different way can we understand our universe. Meanwhile, stating the velocity of our galaxy is meaningless unless it is a velocity against a fixed point, and we have no means of deciding what that point should be.
Hyperfuzzy
5 / 5 (1) Jun 05, 2016
Within every mass of particles there exist a body force and for each particle the near field. On Earth we fall toward the center of the earth. The earth falls toward the center of the sun. The sun falls toward the center of the galaxy. So what is the body force? Or does there exist a superimposed center different from our near field. Go figure. Or is their a collection of galaxies that we exist within? And so on? Jeez, Einstein is wrong! We have out grown that BS.
Reg Mundy
2 / 5 (4) Jun 05, 2016
Within every mass of particles there exist a body force and for each particle the near field. On Earth we fall toward the center of the earth. The earth falls toward the center of the sun. The sun falls toward the center of the galaxy. So what is the body force? Or does there exist a superimposed center different from our near field. Go figure. Or is their a collection of galaxies that we exist within? And so on? Jeez, Einstein is wrong! We have out grown that BS.

There is no body force. All bodies (including us the observers) expand proportionately, which we interpret as theforce of gravity.
Captain Stumpy
2.3 / 5 (3) Jun 11, 2016
All bodies (including us the observers) expand proportionately, which we interpret as theforce of gravity.
1- you failed to prove that here: http://phys.org/n...ong.html

2- this can't define things like tides or tidal forces we can measure, so observation demonstrates this to be false

3- your proportionate claim is false as you stated this
if the mass doubles in size and the acceleration depends on the mass, and the mass doubles in size again, the acceleration doubles again
therefore it would give us a way to measure the acceleration with proportion & size for less dense objects

4- actual physicists reviewed your book and called a whiny self-tribute to your failed life with no empirical evidence or even compelling math capable of supporting your claims

if ya can't get peer reviewed it's because ya aint got evidence
ya can't even figure out how to link a graph here on PO to support your claims
Captain Stumpy
2.3 / 5 (3) Jun 11, 2016
All bodies (including us the observers) expand proportionately, which we interpret as theforce of gravity
@regTROLL
you know, it deserves an answer - you couldn't do it here http://phys.org/n...ong.html

so, logically speaking, if we are all proportionally expanding and there is no gravity, what causes the tides?
and why does it correspond to the Moon movements if you are correct?

what causes the visible effects we measured when D/1993 F2 hit the Roche limit? did it expand itself to death?

what causes orbits around higher mass objects?

given the claim that your expansion is mass dependent, why don't we see higher mass objects grow proportionately larger in shorter periods of time than lower mass similar sized objects?

what would cause gravitational captures we observe if there is no gravity?

observation falsifies your unsubstantiated claims
and your "trust me, i have a book" argument fails 'cause it also has no evidence you can prove
Hyperfuzzy
3 / 5 (2) Jun 13, 2016
Within every mass of particles there exist a body force and for each particle the near field. On Earth we fall toward the center of the earth. The earth falls toward the center of the sun. The sun falls toward the center of the galaxy. So what is the body force? Or does there exist a superimposed center different from our near field. Go figure. Or is their a collection of galaxies that we exist within? And so on? Jeez, Einstein is wrong! We have out grown that BS.

There is no body force. All bodies (including us the observers) expand proportionately, which we interpret as the force of gravity.

Thanks, I thought if i fell to the center of the earth, there would be an infinite force, i.e. 1/r^2. Thanks for clearing that up. You are really smart.
Reg Mundy
2.3 / 5 (3) Jun 13, 2016
Within every mass of particles there exist a body force and for each particle the near field. On Earth we fall toward the center of the earth. ....

There is no body force. All bodies (including us the observers) expand proportionately, which we interpret as the force of gravity.

Thanks, I thought if i fell to the center of the earth, there would be an infinite force, i.e. 1/r^2. Thanks for clearing that up. You are really smart.

While the force is not infinite (except perhaps in a black hole), the pressure caused by all the matter around you expanding would be considerable. Given sufficient matter, the surface of a body expands outwards approaching light speed. Add extra matter, the body cannot expand resulting in a black hole. As time is a function of the expansion, and the body can no longer expand proportionately, it effectively drops out of our universe (becomes a point), the matter within it becoming radiation and fundamental particles.
Captain Stumpy
2.3 / 5 (3) Jun 14, 2016
Given sufficient matter, the surface of a body expands outwards approaching light speed
well that would be something we should be able to see and measure
it would also stand out like a sore thumb
Add extra matter, the body cannot expand resulting in a black hole
WTF?
how does this work?
there are no maths, no charts, no evidence and no anything supporting your claim other than your other claim to "buy your book"

... and considering astrophysicists have read your book and found it lacking in everything except a whiny regurgitation of your life

then it stands to reason that your continued regurgitation of a falsified belief here on PO is simply your own attempt to gain acolytes away from other pseudoscience like eu or flat earthers for the sake of your own narcissistic religion

we'll just call yall the "yeast infection" of the universe, eh?
all rise and expansion, no content but hot air

Captain Stumpy
2.3 / 5 (3) Jun 14, 2016
Thanks, I thought if i fell to the center of the earthblah blah suckup smuchy talk blah ... You are really smart.
WTF?
ROTFLMFAO

so... you are a sock of reg mundy then?

at least now we know for sure!

only reg could comment on how smart he was while being completely incapable of explaining the tides, orbits, lack of any empirical evidence and complete failure of his yeast infection theory that is falsified by observation and evidence
http://phys.org/n...ong.html

so long as the tides are demonstrably pulled by the moon/sun orbits as earth spins there is no possible way reg could be correct

and lets not forget the lack of light-speed growing planets or stars with high mass - and proportional disparities we do not see

Hey smarty-Hyper-fuzzy-reg!
why aren't neutron stars or BH's the size of entire galaxies considering the rate of expansion vrs the universe age of 13 or so billion yrs?
ROTFLMFAO
Hyperfuzzy
3 / 5 (2) Jun 14, 2016
Thanks, I thought if i fell to the center of the earthblah blah suckup smuchy talk blah ... You are really smart.
WTF?
ROTFLMFAO

so... you are a sock of reg mundy then?

at least now we know for sure!

only reg could comment on how smart he was while being completely incapable of explaining the tides, orbits, lack of any empirical evidence and complete failure of his yeast infection theory that is falsified by observation and evidence
http://phys.org/n...ong.html

so long as the tides are demonstrably pulled by the moon/sun orbits as earth spins there is no possible way reg could be correct

and lets not forget the lack of light-speed growing planets or stars with high mass - and proportional disparities we do not see

Hey smarty-Hyper-fuzzy-reg!
why aren't neutron stars or BH's the size of entire galaxies considering the rate of expansion vrs the universe age of 13 or so billion yrs?
ROTFLMFAO

stupid question, presumptive?
Hyperfuzzy
3 / 5 (2) Jun 14, 2016

http://phys.org/n...ong.html

You and the listed author as well as Einstein are wrong. A wavelet emitted only changes frequency due to the media. The media seems to have a constant permittivity and permeability or a small change due to volume; therefore, the wavelet with a period of T_emitted passes a moving observer with T_Observed. The speed is Lambda_emitted divided by T_Observed. You would have to change space and time for Einstein to be correct; which is absurd to anyone who uses their brains and not their ass to think!

Q. E. D.
Hyperfuzzy
3 / 5 (2) Jun 14, 2016
By the way if you sat on a light beam you observe no light beam, juz say'n
Reg Mundy
2.3 / 5 (3) Jun 14, 2016
By the way if you sat on a light beam you observe no light beam, juz say'n

This question may have been intended as a joke, but is actually an acute observation. The ONLY way of detecting photons is to intercept them with our eyes/instruments, so only photons coming directly at us are observable. Think of the zillions of photons that pass us by. They have mass, albeit only tiny or as a product of their energy. Yet we keep looking for dark matter, when it's whizzing by us all the time...
Incidentally, I see the Stumpy twit is contributing his usual drivel, which I see embedded in your previous comment. One of the benefits of phys.org is that they have an "Ignore" facility which I applied to Stumpy long ago. I advise you to do the same...
Reg Mundy
2.3 / 5 (3) Jun 14, 2016
Stumpy, I see you have added Hyperfuzzy to your long, long list of my sock puppets. As far as I can see, you are the only one who persists in using a sock puppet to enhance your ratings, as betrayed by your perpetual 5/5(1) rates - nobody in their right mind would ever rate you above zero....whereas I always get 1/5(1), a badge I wear with pride (I wonder who always rates me instantly as 1... could it be you?).
By the way, don't bother replying to this, you are on my ignore list so I won't read it anyway.
Reg Mundy
2.3 / 5 (3) Jun 14, 2016

You and the listed author as well as Einstein are wrong. A wavelet emitted only changes frequency due to the media. The media seems to have a constant permittivity and permeability or a small change due to volume; therefore, the wavelet with a period of T_emitted passes a moving observer with T_Observed. The speed is Lambda_emitted divided by T_Observed. You would have to change space and time for Einstein to be correct; which is absurd to anyone who uses their brains..

Right H., change space and time, but whilst the Establishment insist that light travelling out of a gravity well loses energy manifest in a longer wavelength, I believe that in an expanding universe (of which we are part, also expanding) as time elapses we move faster and faster away from the point of origin of any light and therefore experience it with a longer wavelength. The further it has travelled the longer the time the faster we recede hence the longer the wavelength. No gravity!
Hyperfuzzy
3 / 5 (2) Jun 14, 2016
By the way if you sat on a light beam you observe no light beam, juz say'n

This question may have been intended as a joke, but is actually an acute observation. The ONLY way of detecting photons is to intercept them with our eyes/instruments, so only photons coming directly at us are observable. Think of the zillions of photons that pass us by. They have mass, albeit only tiny or as a product of their energy. Yet we keep looking for dark matter, when it's whizzing by us all the time...
Incidentally, I see the Stumpy twit is contributing his usual drivel, which I see embedded in your previous comment. One of the benefits of phys.org is that they have an "Ignore" facility which I applied to Stumpy long ago. I advise you to do the same...

There are no photons, this came from Einstein's corpuscular theory, i.e. BS
Hyperfuzzy
3 / 5 (2) Jun 14, 2016
No gravity!

First, better measurements; then, theory.
Hyperfuzzy
3 / 5 (2) Jun 14, 2016
Note: Before saying photons and other nonsense, recall that the field of each particle is a spherical field, beams are a focused phenomenon, not particle gathering. The slit experiment require we have a thoughtful review. All particles have this field, whether the particle enters the slit or not. The field is the evidence, not a duality. Duality is nonsense. This is obvious, even to an unintelligent creature like myself! Also there are only two particles, one with a + charge and the other a - charge. Anti-matter is a fabrication of a confused mind. re: cloud chamber, same path, yet mirror image, get it, opposite charge, not anti-? Time to ignore graduates with a PhD in Theoretical Physics of the imagination and stupid experiments with mis-interpretations.
Reg Mundy
2.3 / 5 (3) Jun 14, 2016
H
there are only two particles, one with a + charge and the other a - charge

I have long claimed this to be true, except that every - charge would be surrounded by virtual + charges, and vice versa. The smallest charge known to science (currently) is a third of an electron. So I named my particles tirds for fun, and call the tird with the - sign a t-rd and the positive one a t+rd. I don't mind thinking of the smallest combo (1 t-rd and 1 t+rd) as a photon. Its an easy way to refer to the combo. And of course the orbital time provides the frequency/wavelength, the elliptical orbits the velocity due to relativistic forces, and this being two bodies it is two-dimensional, hence polarity. All this, plus "no gravity", is laid out logically in my book no matter what Stumpy says, he is too thick to follow the logic.
Incidentally, we can still call an atom an atom even tho' we know it is made up of smaller fundamental particles, which are made up of smaller ..and so on to tirds
Captain Stumpy
2.3 / 5 (3) Jun 15, 2016
stupid question
@Hyper-mundy
is it? reg states
Given sufficient matter, the surface of a body expands outwards approaching light speed
this is something that would be an observable test of his belief, and we do *not* see this in the universe-anywhere
You and the listed author as well as Einstein are wrong
and if you could prove that you would ha a Nobel award winner
... added Hyperfuzzy to your long, long list of my sock puppets
i didn't know you had a list of socks, but now that you admitted as much i can start looking for more, thanks

considering the compliments based upon the above, it demonstrates either insanity or sock conversation

you can't answer ANY of the points above but you still think you have a viable hypothesis?
why?

is it because of the way the books are flying off the shelf?
(snicker)

PS-never used a sock like you cause i've never promoted bullsh*t, lies or pseudoscience like you
Captain Stumpy
2.3 / 5 (3) Jun 15, 2016
@hyper-mundy cont'd
(I wonder who always rates me instantly as 1... could it be you?
i rate you based on your content: as you promote pseudoscience and you can't demonstrate your point with evidence, references or links then i will rate you with a 1 when i see you promote your failed ideas

you can't produce anything except for "read my book" which has also been reviewed by astrophysicists and found to have all the same evidence as the bible -none
By the way, don't bother replying to this, you are on my ignore list so I won't read it anyway
i thought i already was on your ignore list... so why did you reply?

did you read it while logged in as a sock spreading your pseudoscience crap?

the only reason you are still here promoting your BS is because PO refuses to actually moderate the trolls/pseudoscience off the site

at least now you have a place where you can be popular and considered not a crank by the other cranks?
LMFAO

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