Last year, Sasha Kashlinsky of the Goddard Space Flight Center in Greenbelt, Maryland, and colleagues identified an unusual pattern in the motion of around 800 galaxy clusters. They studied the clusters' motion in the "afterglow" of the big bang, as measured by the Wilkinson Microwave Anisotropy Probe (WMAP). The photons of this afterglow collide with electrons in galaxy clusters as they travel across space to the Earth, and this subtly changes the afterglow's temperature.
The team combined the WMAP data with X-ray observations and found the clusters were streaming at up to 1000 kilometres per second towards one particular part of the cosmos.
Their latest analysis reveals 1400 clusters are part of the flow, and that it continues to around 3 billion light years from Earth, a sizeable fraction of the distance to the edge of the observable universe (arxiv.org/abs/0910.4958). This is twice as far as seen in the previous study.
(15 page pdf) A new measurement of the bulk flow of X-ray luminous clusters of galaxies
We present new measurements of the large-scale bulk flows of galaxy clusters based on 5-year WMAP data and a significantly expanded X-ray cluster catalogue. Our method probes the flow via measurements of the kinematic Sunyaev-
Zeldovich (SZ) effect produced by the hot gas in moving clusters. It computes
the dipole in the cosmic microwave background (CMB) data at cluster pixels,
which preserves the SZ component while integrating down other contributions.
Our improved catalog of over 1,000 clusters enables us to further investigate possible systematic effects and, thanks to a higher median cluster redshift, allows us to measure the bulk flow to larger scales. We present a corrected error treatment
and demonstrate that the more X-ray luminous clusters, while fewer in number,
have much larger optical depth, resulting in a higher dipole and thus a more
accurate flow measurement.
Our recent discovery of a coherent large-scale flow of galaxy clusters with significantly larger amplitude than expected out to ' 300Mpc (Kashlinsky et al 2008, 2009 – KA-BKE1,2) represents a challenge to the gravitational instability paradigm. Such a ”dark flow” could be indicative of a tilt created by the pre-inflationary inhomogeneous structure of spacetime (Turner 1991, Grischuk 1992, Kashlinsky et al 1994, KA-BKE1) and might provide an indirect probe of the Multiverse. Various explanations for the flow have been put forward, including that the flow points to a higher-dimensional structure of gravity (Afshordi et al 2009, Khoury & Wyman 2009), or that it reflects the pre-inflationary landscape produced by certain variants of string cosmology (Mersini-Houghton & Holman 2009, Carrol et al 2008). Making use of an expanded cluster catalog and deeper WMAP observations, we have worked to verify, and expand the Dark Flow study.
Nosey neighbours : A Related but Separate Research
(8 page pdf) ’Tilting’ the Universe with the Landscape Multiverse: The ’Dark’ Flow
The theory for the selection of the initial state of the universe from the landscape multiverse predicts superhorizon inhomogeneities induced by nonlocal entanglement of our Hubble volume with modes and domains beyond the horizon. Here we show these naturally give rise to a bulk flow with correlation length of order horizon size. The modification to the gravitational potential has a characteristic scale L1 ≃ 103H−1, and it originates from the preinflationary remnants of the landscape. The ’tilt’ in the potential induces power to the lowest CMB multipoles, with the dominant contribution being the dipole and next, the quadrupole. The induced multipoles l ≤ 2 are aligned with an axis normal to their alignment plane being oriented along the preferred frame determined by the dipole. The preferred direction is displayed by the velocity field of the bulk flow relative to the expansion frame of the universe. The parameters are tightly constrained thus the derived modifications lead to robust predictions for testing our theory. The ’dark’ flow was recently discovered by Kashlinsky et al. to be about 700km/s which seems in good agreement with our predictions for the induced dipole of order 3μK. Placed in this context, the discovery of the bulk flow by Kashlinsky et al. becomes even more interesting as it may provide a probe of the preinflationary physics and a window onto the landscape multiverse.
Was our universe once entangled with a neighbour? The observation of "dark flow" in galaxy clusters was predicted in 2006 by Laura Mersini-Houghton of the University of North Carolina at Chapel Hill and colleagues. She proposes that the effect occurs because our universe was once influenced by neighbouring domains (arxiv.org/abs/0810.5388).
Mersini-Houghton reasoned that if a force exerted by other universes squeezed ours, it could generate a repulsive effect that would impede the shrinkage of matter into clusters but not leave an imprint on smaller scales. "This skews the distribution of lumps so they are not the same in all directions," she says. "There is a preferred direction - the dark flow."
She also predicted in 2006 that there should be two "holes" - regions with fewer galaxies than expected. Sure enough, there does appear to be a hole - the so-called "cold spot" identified by the WMAP probe. The hole is a very large region of space where the afterglow is cooler than average. However, its cause - and even existence - is disputed, and Mersini-Houghton's hypothesis remains controversial.