Dark Energy, Dark Flow, and Can We Explain It Away?
by Ethan Siegel
Starts With A Bang
October 6, 2011
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"Deep into that darkness peering, long I stood there,
wondering, fearing, doubting, dreaming dreams no mortal
ever dared to dream before." -Edgar Allen Poe
Earlier this week, the Nobel Prize in Physics was
announced for the discovery that the Universe is not
only expanding, but that this expansion is accelerating!
What does an accelerated expansion physically mean?
If all you had in the Universe was some initial
expansion and the mutual gravitational attraction of
everything in it, you'd expect that as an object got
farther and farther away from you, over time, its
apparent motion away from you would slow down.
(Image credit: HowStuffWorks.)
If there was enough matter, you'd expect that the
expansion would eventually lose out to gravity, and that
the objects moving away from you today would someday
reverse course and wind up moving towards you.
If there weren't enough, the expansion would win out,
and that the objects moving away from you today would
slow down some, but would keep on moving away from you
for all eternity.
But if the expansion is accelerating, there's something
more to it.
(Image credit: Kyle Dawson.)
Unlike the three decelerating cases above (with O > 0,
where the recession speed of any particular galaxy slows
down over time), or even the case of an empty Universe
(with O = 0, where the recession speed of a galaxy
remains constant), a Universe with an accelerated
expansion will actually have a particular galaxy recede
away from you faster and faster as time goes on!
(For some more details, check out Sean Carroll's dark
Until the 1990s, it was pretty much assumed that the
Universe would be decelerating, and it was thought that
in order to understand both the history and fate of the
Universe, there would be two important measurements we'd
have to make.
(Image credit: HST Key Project, Freedman et al. 2001.)
The first would be Ho, the Hubble parameter today. If a
galaxy is a certain distance away from us, we expect to
find it moving away from us at a certain rate, where the
apparent recessional speed is given, very simply, by
Hubble's Law. For relatively nearby objects (i.e.,
galaxies "only" about a billion light-years or less away
from us), accelerating, decelerating and empty Universes
all look the same.
(Image credit: Knop et. al. 2003, ApJ, 598, 102.)
But the second important measurement (known as qo, which
is traditionally called the deceleration parameter)
tells us whether the Universe is accelerating or
decelerating, and it is very sensitive to the motions of
faraway objects! In the figure above, the lowest line
has a deceleration parameter of qo = +½, the middle line
has qo ~ +0.1, and the top line -- the best fit to the
cosmological data from our actual Universe -- has qo ~
-0.6. (I don't normally like to talk numbers, but these
are going to be important later, so remember it; the
best fit to our Universe's data has a deceleration
parameter of qo ~ -0.6.)
That negative value tells that the Universe is not
decelerating at all, but is in fact accelerating in its
expansion! And we learned this, of course, from looking
at those very bright, well-known objects that are so
visible at such large distances: supernovae!
(Image credit: NASA/ESA, The Hubble Key Project Team and
The High-Z Supernova Search Team.)
There have been attempts in the past to explain these
observations with something other than an accelerated
expansion, and I've written many articles detailing how
But recently, there's been a new idea out there getting
some press; that something called dark flow could be
mimicking an accelerated expansion!
(Link to this space.com story.)
Cosmologist Christos Tsagas has written a paper (that's
actually a follow-up to an earlier one) that notes
something very clever.
You see, Hubble's Law -- the nearby relationship between
an object's apparent recessional speed and its distance
-- is true on average, but it is not a good predictor of
any individual object's speed.
(Image credit: T. Sanchis et al., 2004.)
You see, in addition to getting swept up in the
expansion of the Universe, every object is subject to
local gravitational forces, that gives it an extra
motion on top of the Hubble expansion, known as a
peculiar velocity. It should come as no surprise that
not only do we observe this, but simulations predict it
(Video credit: Ralf Kaehler.)
In fact, it's well-measured that, relative to the
uniform temperature surface of the cosmic microwave
background, our galaxy has a substantial peculiar
velocity of about 627 km/s, which is actually huge:
about 1.4 million miles per hour!
(Image credit: DMR, COBE, NASA, Four-Year Sky Map.)
Now, this dipole is not exactly indicative of our
peculiar velocity. They ought to be related, of course,
but because the Earth orbits the Sun, the Sun is
orbiting the galaxy, and our galaxy is constantly being
tugged on by all the others in the Universe, this
peculiar motion will actually change somewhat over time!
(Animation credit: J.P. Zibin, Adam Moss and Douglas
We have, in fact, mapped out the peculiar velocities of
a great many objects in our neighborhood. What we've
found is that not only do many of these objects cluster
together in small groups which move together, but that
there is an unexpectedly large overall dark flow on
scales hundreds of million of light years in size!
(Image credit: G. Theureau et al., 2007.)
Now, here's where Tsagas' idea comes in, and it's very
clever. He finds that if you're moving relative to the
CMB rest frame, this relative motion will cause your
local region of space to have a different expansion rate
from the overall Universe!
This should have the largest effect in the nearby
Universe, because as you move to larger and larger
scales, your peculiar velocity, even if it's thousands
of km/s, will eventually become negligibly small
compared to the overall Hubble expansion.
Now remember, far above, we said that the best fit of
our models to the data show that the deceleration
parameter, qo ~ -0.6, although at very much earlier
times -- when dark energy was unimportant -- the
Universe was, in fact, dominated by matter, and
decelerating with an approximate deceleration parameter
of qo ~ +½.
(Image credit: Figure 1 from C. Tsagas, 2011.)
Now, the big question, of course, is whether Tsagas'
model can explain the same cosmological data in the
supernovae that dark energy does. And to Tsagas' great
credit, he is honest about what his results give, and
how they compare to our concordance cosmology.
The answer is no. His model can give apparent
deceleration parameters as negative as about qo ~ -0.3,
but no more than that. And that deceleration parameter
goes approximately towards zero very quickly, and
becomes small and positive (but much less than the qo ~
+½ predicted by the standard Lambda-CDM model) by z =
0.3 at the latest. Now, I worried that this would become
very problematic at the intermediate redshifts, where
the support for the dark energy model is strongest.
So, I took the best available supernova data, along with
the fits for models such as the dark energy model, an
empty Universe, and a few others (from Ned Wright's
site), and I put in a line to try to fit my best
calculations for Tsagas' model.
(Image credit: Ned Wright 2011, with the orange line
from me using Tsagas' data.)
Now, I do give Tsagas' model a ton of credit for being
able to produce a significant (if not quite sufficient)
early rise in that curve, because it does so without
invoking any dark energy or negative-pressure fields!
But the low-density nature of Tsagas' toy Universe winds
up producing predictions that are too close to the
"empty Universe" model that are inconsistent with the
So it's a neat little toy, and it's very impressive that
it produces any sort of positive acceleration, but it
doesn't look like it can replace dark energy. Still,
it's always worth exploring alternatives, and every time
I do, I find myself a little more convinced of how
impressively dark energy actually works!
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