Everybody's been talking about the new and mysterious "dark flow" that's supposedly tugging on galaxies from beyond the edge of the observable universe. Cosmologist Niayesh Afshordi wonders if it really deserves the hype.
From Niayesh Afshordi:
It is no secret that modern cosmologists have developed an obsession with the dark sciences; or at least, that's what you may think if you tuned into popular science articles over the past couple of decades. What's ironic is that most of these obsessions have started through the study of clusters of galaxies or gigantic cosmic lighthouses which make up the most luminous objects in the Universe.
It started with Fritz Zwicky, the eccentric astronomer from CalTech, who noticed some 70 years ago that in order to keep the Coma galaxy cluster bound together, its mass must be several hundred times larger than the mass of stars in its galaxies. Otherwise, the galaxies of the cluster would simply fly away in a short amount of time, and the cluster wouldn't exist now. This led to the Dark Matter hypothesis, which has been with us ever since. The case for it has only gotten stronger over time.
Then came Dark Energy, some 15 years ago, when astronomers noticed that the number of galaxy clusters doesn't seem to be increasing as quickly as simple models would predict. The fact is that in a universe full of dark matter, you'd expect to keep forming bigger and bigger structures (and thus bigger galaxy clusters) because there'd be nothing to keep more dark matter from falling into the structures.
But this isn't the case. The number of clusters of a given mass remarkably showed little change over the past several billion years. This eventually led to the conclusion that the expansion of the universe is accelerating due to the recent dominance of some mysterious dark energy. Similar to dark matter, the dark energy hypothesis has been confirmed through other independent observations, the most significant of which were the observations of distant, high redshift Type Ia Supernovae around the turn of the century.
Now it turns out that cosmic lighthouses might have more surprises in store for us. About a week ago, two preprints by Kashlinsky, Atrio-Barandela, Kocevski, and Ebeling, found their way onto the physics arXiv and then into the Astrophysical Journal, and in the process captured the headlines on every single news outlet on the web, including NASA site's, The Telegraph, and even Fox News (no surprises in their obsession with the dark side!).
So what is this mysterious "Dark Flow" that all of a sudden everybody seems to be talking about, and despite being a cosmologist I first heard about only last week? My first reaction is that cosmologists have learnt their lesson in marketing: that people love *everything* *Dark*. (Although sadly the title of my job talks last year, "Journey into the Dark Side," yielded little success!)
But more seriously, Kashlinsky et al. have examined data from the cosmic microwave background--the relic radiation left behind by the big bang--measured by the Wilkinson Microwave Anisotropy Probe (WMAP). They noticed that a specific microwave signal from galaxy clusters (known the kinetic Sunyaev-Zeldovich effect) observed in WMAP microwave maps is consistent with the idea that everything out to a billion light years from us is moving coherently with the velocity of ~ 1000 km/s in some mysterious direction (see image, right). This speed is around 10 times larger than the standard cosmological model with dark energy would predict. Since they cannot explain the motion with anything within the observable uinverse, they go on to speculate that matter from beyond the edge of the observable universe--some pre-inflationary super structure--could possibly be tugging on the galaxies and causing this super-Dark flow.
Ok. All this is fine and good. In fact, if this result holds up, it would be the first detection of the kinetic Sunyaev-Zeldovich effect in cosmology, which would be very impressive. Moreover, my colleague Mike Hudson at the University of Waterloo and his collaborators, have recently submitted a preprint that obtains a similar dark flow on slightly smaller scales, but from a completely independent method using distance measurements to optical galaxies.
But then there also seem to be some disconcerting points in Kashlinsky et al's papers, which may cast doubt on how reliable the analysis might be. Without getting too deep into technical detail, my main concerns have mostly to do with their over simplistic model for gas distribution in galaxy clusters (known as the beta model), which has led to erroneous conclusions in the past. (In fact, I discussed this problem in an article in USA Today a couple of years ago, when a different study concerning the Sunyaev-Zeldovich effect prematurely called the Big Bang model into question.) Moreover, X-ray luminosities that are used here to calibrate gas distribution are known to be affected by complicated astrophysical processes at the cluster centers, introducing another systematic uncertainty into the analysis. On a different note, the WMAP data under analysis is split into bands of different frequency, and here the authors appear to use a naïve combination of their results in these different bands (ignoring the correlation between different frequencies), which seems to give unrealistically small errors.
We shall see whether future work will address, or rather reinforce, my concerns. Until then, thanks to those cosmic lighthouses, we might be glimpsing pre-inflationary super-structures through this new, cosmic dark flow. Or this may be an over simplistic interpretation of another (still absolutely beautiful) natural phenomenon.
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Niayesh Afshordi is a cosmologist and WMAP expert at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario.