Archive for October, 2016

Dark Matter – An Update

October 9, 2016

In Genesis and Genes, I devoted some space to the notion of Dark Matter. I recently read an article in Nature about developments in this area, and I’d like to update my readers about this fascinating subject.

What follows is an excerpt from Genesis and Genes (for the purpose of this post, I have omitted the endnotes that appear in the book). I will then comment on the article in Nature.

&&&

Nobody – including astronomers and cosmologists – knows what the universe is made of. Visible matter – the kind of stuff that people and planets are made of – is outweighed by a factor of 6 or 7 by invisible, cold dark matter. To put it another way, something like 95% of the universe is made up of stuff we can’t detect, except that it seems to exert a gravitational pull. Here is how one distinguished astronomer and author, James Kaler, puts it:

Our Galaxy, its stars revolving around the center under the influence of their combined gravity, is spinning too fast for what we see. Galaxies in clusters orbit around the clusters’ centers under the influence of their mutual gravities, but again, they move faster than expected. There must be something out there with enough of a gravitational hold to do the job, to speed things up, but it is completely unseen. Dark matter… We have no idea what constitutes it. Rather, there are many ideas, but none that can be proven.

A popular history of astronomy weighs in with this:

Over 90 per cent of our Universe is invisible – filled with particles of mysterious dark matter. And astronomers have no idea what it is. Theoretical physicists working on the kinds of particles produced in the Big Bang say that dark matter cannot be anything ordinary – it has to be something very exotic.

I don’t wish to labour the point, but I must. The public is subjected to absolute statements about our knowledge of the universe and its history so frequently that the average person is simply inured to the fact that there remain basic questions about our cosmic abode. To wit, we do not know what it is made of. Consider this. The most ambitious project in astronomy in the early 21st century is the SKA, or Square Kilometre Array, a network of radio telescopes that is gargantuan in every respect: complexity, size and cost. An article in TIME magazine about the instrument begins by asking the project manager what it is that astronomers wish to discover with this machine:

For someone whose job title could read Man Most Likely to Blow Your Mind, Bernie Fanaroff looks pretty conventional… Consider the fact, says Fanaroff, that we have no idea what 96% of the universe is made of. Cosmologists have known for some time that only 4% of the universe is stuff like dust, gas and basic elements. Dark matter, says Fanaroff, accounts for 23% to 30%; dark energy makes up the rest. (Dark, Fanaroff explains, is the scientific term for “nobody knows what it is.”)

That’s not an exaggeration – nobody knows anything significant about what makes up 96% of the universe. And this is acknowledged even by those who pretend to be able to answer ultimate questions in naturalistic terms. Lawrence Krauss is a world-famous physicist and an ardent atheist. His latest book, A Universe from Nothing: Why There Is Something Rather than Nothing (Free Press, 2012) was reviewed in the January 2012 issue of Nature, the world’s most respected science journal. Nature appointed Caleb Scharf, an astrobiologist at Columbia University, to aggrandise Krauss’s ideas about the universe popping out of absolutely nothing, but even he could not hide the gigantic lacuna in Krauss’s thesis:

He notes that a number of vital empirical discoveries are, ominously, missing from our cosmic model. Dark matter is one. Despite decades of astrophysical evidence for its presence, and plausible options for its origins, physicists still cannot say much about it. We don’t know what this major mass component of the Universe is, which is a bit of a predicament. We even have difficulty accounting for every speck of normal matter in our local Universe.

It is crucial to appreciate that dark matter is not something that was initially discovered in a laboratory, and whose existence was then used to explain some phenomenon. It is also not an entity whose existence was implied by some cosmological theory, and then applied to the problem of energetic stars. Dark matter is entirely hypothetical. Its existence was postulated to explain how the stars in spiral galaxies can orbit at such breakneck speeds without being flung off into the void. In other words, when astronomers tallied up all the mass in the universe, they came face to face with a phenomenon which they could not explain using known physical laws: those laws would indicate that stars in spiral galaxies should indeed be flying off in all directions. Since they aren’t, there must be something out there to prevent them from doing so. What that something is remains anybody’s guess, as Professor Kaler pointed out above. Many astronomers believe that there is matter out there; matter which for whatever reason, we cannot see. This is why they refer to this hypothetical entity as dark matter. They appear to have considerable fun in speculating on the nature of this hypothetical matter: is it made up of MACHOs (Massive Compact Halo Objects)? Or is it WIMPs (Weakly Interacting Massive Particles)?

But since the whole exercise is built on speculation as to what could possibly be acting as a brake on those wayward stars, other scientists do not believe that dark matter even exists. And there is nothing to contradict their view. All you have to do is propose a plausible mechanism to restrain energetic stars from flying off into the cosmic sunset. [END OF QUOTATION FROM GENESIS AND GENES.]

&&&

A recent article in Nature, written by Jeff Hecht and cleverly entitled Dark Matter: What’s the Matter? provides a welcome update in this regard.[1] Hecht begins by introducing the subject:

Most of the Universe is missing. The motion of the stars and galaxies allows astronomers to weigh it, and when they do, they see a major discrepancy in cosmological accounting. For every gram of ordinary matter that emits and absorbs light, the Universe contains around five grams of matter that responds to gravity, but is invisible to light. Physicists call this stuff dark matter, and as the search to identify it is now in its fourth decade, things are starting to get a little desperate.

A little later, Hecht discusses a new attempt to crack the problem, one that has both supporters and detractors within the scientific community. Hecht is not optimistic about the latest approach:

It looks unlikely that primordial black holes are the mysterious dark matter. And as time passes without a confirmed detection, even the most heavily backed theories are beginning to look less likely. A series of experiments have systematically searched for, and failed to find, the theoretical candidates for dark matter — one by one, the possibilities are being reduced. A raft of experiments designed to finally detect, or refute, the remaining candidates are now underway, each with vastly different approaches to the problem. As more options are crossed off the list, physicists may have to explore new ideas and reconsider alternative theories… — or accept that nature may have hidden dark matter just out of our reach.

When Genesis and Genes was written, MACHOS – massive Compact Halo Objects – were still considered candidates for Dark Matter. No longer:

Decades of research have narrowed down the possibilities. Early favourites included not only black holes, but also other massive compact halo objects (MACHOs) made of ordinary matter. A series of studies, however, gradually ruled out most of the possibilities… But in the view of theoretical physicist John Ellis of King’s College London, “MACHOs are dead.”

The other candidate for Dark Matter I mentioned in Genesis and Genes was WIMPS – Weakly Interacting Massive Particles. WIMPS still hold some promise for resolving the Dark Matter conundrum:

Although MACHOs have fallen by the wayside, another candidate has hung around. A decade ago, physicists were largely convinced that dark matter was made up of weakly interacting massive particles (WIMPs)…

WIMPs remain the leading candidate for dark matter. “Supersymmetry is beautiful mathematically,” says physicist Oliver Buchmueller of Imperial College London. “With just one weakly interacting particle, we can explain all the dark matter we see in the Universe.” Indeed, so well does the lightest of these hypothetical particles fit the bill for dark matter that it has been called “the WIMP miracle”, says physicist Leslie Rosenberg of the University of Washington in Seattle.

But only in theory:

But supersymmetrical particles have proved maddeningly elusive. Physicists at CERN, Europe’s particle-physics laboratory, are searching for WIMPs with the Large Hadron Collider (LHC) by smashing protons or atomic nuclei together to recreate the conditions of the early Universe… The longer the puzzle goes unsolved, the more twitchy the scientific community will become. “People are a little nervous,” says Rosenberg.

Hecht goes on to discuss the difficult – and rather exotic – ways in which scientists use particle colliders to try to detect recalcitrant particles:

Researchers won’t see dark matter directly. Instead, they look for signs that energy and momentum in collisions have gone missing when they should have been conserved. Ellis compares searching for evidence of dark matter to watching billiard balls roll away after the cue ball hits them on the break shot. If the balls on one side of the group were invisible, and only the balls rolling away on the opposite side could be seen, the path and nature of the unseen balls can still be deduced, he says. Physicists are using the paths of the particles they can see to identify the paths of the dark matter that they can’t.

So far, nothing has come up.

Dark Matter is a fascinating scientific problem. For informed consumers of science, a number of issues are important in this context:

  1. We don’t know what 95% of the universe is made of! That’s astonishing. Members of the public should be aware that when peremptory remarks about the universe are made by scientists, or in magazine articles, or in documentaries, they hide enormous assumptions about how much we really know. As I explain in Genesis and Genes, Dark Matter (and Dark Energy) may one day turn out to be made of exotic particles; then again, it is quite possible that the scientific picture of our universe is seriously wrong, a possibility freely acknowledged by astronomers such as James Kaler and physicists like Mordechai Milgrom. Don’t be duped by those who insist that matter and energy form the fundamental substrate of our universe. This view originates in an ideology – scientism – and not in evidence from Nature itself. The only reasonable response to knowing how little we know about the universe is humility.

2. It is worth bearing in mind the similar situation that pertained in biology before the Junk DNA paradigm collapsed (see my previous post, Francis Collins Does Teshuva). In that context, many biologists dismissed about 95% of the human genome as junk, because they did not know what it did. This turned out to be a spectacular failure, delaying by several decades the onset of the age of epigenetics. In my view, physicists and astronomers are generally more open to the possibility of paradigm shifts than are biologists. They are also more likely to admit, in public, that major lacuna remain in our knowledge of the physical world.

 3. All the methods that have been devised to detect Dark Matter rely on complicated statistical analyses to infer particles of Dark Matter. This is not a simple matter of observation, and lends itself to different interpretations. Here, too, the history of science would indicate that healthy scepticism be maintained when certain results are proclaimed.

REFERENCES:

[1] http://www.nature.com/nature/journal/v537/n7622_supp/full/537S194a.html

Retrieved 7th October 2016.

Advertisements