Blobs of ancient dark matter shed light

Through the light of ancient galaxies astronomers can watch the evolution of the universe like a movie, but one that’s missing its opening scene – the Big Bang itself.  

One of the great challenges in cosmology is to try to piece together what happened in that dramatic opener, by studying any of its original participants left behind.

Now physicists have found a new way to test for one of the central theories of how the universe was born – the idea of inflation – by looking for mini clumps of dark matter which could still be dotted between galaxies.

According to the new work led by Grigor Aslanyan at the University of Auckland in New Zealand, blobs of primordial dark matter (small on a cosmological scale – much smaller than galaxies) could have been created in great numbers in the first split-second of the universe.

Physicists may be able to spy the clumps through how they distort the light of distant celestial objects. The size and number of them could tell us much about the Universe’s missing opening scene.

The work, published in Physical Review Letters – links two of the great mysteries of the universe: dark matter and inflation.

Dark matter is the mysterious “stuff” that makes up most of the material in the universe – but which physicists are struggling to identify.

Inflation is the idea that the universe experienced a sudden growth spurt just after the Big Bang, ballooning from the size of a proton to a grapefruit.

In that split-second, the universe experienced more expansion, in terms of relative size, than it has in all the billions of years since.

And while inflation seems a wild idea, most physicists accept it because it satisfies many cosmological head-scratchers. But there is as yet no definitive proof of inflation, and nobody knows what caused it – or why.

The problem is astronomers peering back at the earlier universe hit a wall. The earliest light they can detect comes from what’s called cosmic microwave background radiation.

Often described as the afterglow of the Big Bang, the cosmic microwave background was actually emitted some 300,000 years after the event when universe’s hot early plasma began to clear up.

To figure out what happened before this time, physicists have to look for clues left behind in the structure of the universe. 

Physicists think this structure originated as tiny quantum fluctuations which were blown up through inflation to form slight variations in density – a cosmic “lumpiness” that went on to define the distribution of matter everywhere.

Larger lumps turned into huge superclusters of galaxies threaded together in a cosmic web. Smaller lumps formed individual galaxies.

And, according to the new work, smaller lumps still may have formed small dense objects: clumps of dark matter smaller than galaxies.

Although we can’t see the dark matter directly, mini-clumps of the stuff should be detectable by how their gravity distorts space and time.

These “dark matter minihalos” could still be out there dotting the voids between galaxies. Mapping their number and size could give vital information about when went down during inflation.

The challenge is in detecting them.

Although we can’t see the dark matter directly, mini-clumps of the stuff should be detectable by how their gravity distorts space and time.

As light passes through a gravitational field, such as that surrounding a blob of dark matter, its path can be slowed. This can have a noticeable effect on the timing of pulsars – rotating neutron stars that emit a beam of radiation like a lighthouse – making their regular ticking appear to momentarily slow.

Astronomers have been watching pulsars for decades and so far have not picked up any momentary slow-ticks. But this doesn’t mean they are not there – just that they’re too small to be detected, and techniques for monitoring pulsars are improving all the time.

Soon astronomers should be able to detect smaller and smaller dark matter clumps. 

Even this non-detection can tell us much about the universe’s earliest growth spurt, argue Aslanyan and his team. It places a limit on the lumpiness of the universe which physicists can feed into their model of inflation.

In a Viewpoint piece for the American Physical Society, David Parkinson, an astrophysicist at the University of Queensland in Australia, said that the research “significantly widens the available observational pathways to understand the early universe, and so may provide a future key piece of information as to exactly how inflation took place”.

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