Is dark matter cold, warm or hot? + Did Venus, Earth's twin sister, ever have oceans?

C C Offline
Is dark matter cold, warm or hot?

INTRO: Half a century after Vera Rubin and Kent Ford confirmed that a form of invisible matter—now called dark matter—is required to account for the rotation of galaxies, the evidence for its existence is overwhelming.

Although it is known to interact with ordinary matter only through gravity, there is such a massive amount of dark matter out there—85% of all the matter in the universe—that it has played a pivotal behind-the-scenes role in shaping all the stuff we can see, from our own Milky Way galaxy to the wispy filaments of gas that link galaxies across vast distances.

“We think it exists because there’s evidence for it on many, many scales,” says Kevork Abazajian, a theoretical physicist and astrophysicist at the University of California, Irvine.

There have been a lot of ideas about what form dark matter might take, from planet-sized objects called MACHOs to individual particles like WIMPs—weakly interacting massive particles roughly the size of a proton—and even tinier things like axions and sterile neutrinos.

In the 1980s, scientists came up with a way to make sense of this growing collection: They started classifying proposed dark-matter particles as cold, warm or hot. These categories are based on how fast each type of dark matter would have traveled through the early universe—a speed that depended on its mass—and on how hot its surroundings were when it popped into existence.

Light, fast particles are known as hot dark matter; heavy, slow ones are cold dark matter; and warm dark matter falls in between. In this way of seeing things, WIMPs are cold, sterile neutrinos are warm, and relic neutrinos from the early universe are hot. (Axions are a special case—both light and extremely cold. We’ll get to them later.)

Why is their speed so important? “If a dark matter particle is lighter and faster, it can travel farther in a given time, and it will smooth out any structure that already exists along the way,” Abazajian says. On the other hand, slower, colder forms of dark matter would have helped build structure, and based on what we know and see today it must have been part of the mix... (MORE)

Did Venus, Earth's twin sister, ever have oceans?

RELEASE: The planet Venus can be seen as the Earth’s evil twin. At first sight, it is of comparable mass and size as our home planet, similarly consists mostly of rocky material, holds some water and has an atmosphere. Yet, a closer look reveals striking differences between them: Venus’ thick CO2 atmosphere, extreme surface temperature and pressure, and sulphuric acid clouds are indeed a stark contrast to the conditions needed for life on Earth.

This may, however, have not always been the case. Previous studies have suggested that Venus may have been a much more hospitable place in the past, with its own liquid water oceans. A team of astrophysicists led by the University of Geneva (UNIGE) and the National Centre of Competence in Research (NCCR) PlanetS, Switzerland, investigated whether our planet’s twin did indeed have milder periods. The results, published in the journal Nature, suggest that this is not the case.

Venus has recently become an important research topic for astrophysicists. ESA and NASA have decided this year to send no less than three space exploration missions over the next decade to the second closest planet to the Sun. One of the key questions these missions aim to answer is whether or not Venus ever hosted early oceans.

Astrophysicists led by Martin Turbet, researcher at the Department of Astronomy of the Faculty of Science of the UNIGE and member of the NCCR PlanetS, have tried to answer this question with the tools available on Earth. “We simulated the climate of the Earth and Venus at the very beginning of their evolution, more than four billion years ago, when the surface of the planets was still molten”, explains Martin Turbet. “The associated high temperatures meant that any water would have been present in the form of steam, as in a gigantic pressure cooker.”

Using sophisticated three-dimensional models of the atmosphere, similar to those scientists use to simulate the Earth’s current climate and future evolution, the team studied how the atmospheres of the two planets would evolve over time and whether oceans could form in the process. “Thanks to our simulations, we were able to show that the climatic conditions did not allow water vapour to condense in the atmosphere of Venus”, says Martin Turbet.

This means that the temperatures never got low enough for the water in its atmosphere to form raindrops that could fall on its surface. Instead, water remained as a gas in the atmosphere and oceans never formed. “One of the main reasons for this is the clouds that form preferentially on the night side of the planet. These clouds cause a very powerful greenhouse effect that prevented Venus from cooling as quickly as previously thought”, continues the Geneva researcher.

Small differences with serious consequences. Surprisingly, the astrophysicists’ simulations also reveal that the Earth could easily have suffered the same fate as Venus. If the Earth had been just a little closer to the Sun, or if the Sun had shone as brightly in its ‘youth’ as it does nowadays, our home planet would look very different today. It is likely the relatively weak radiation of the young Sun that allowed the Earth to cool down enough to condense the water that forms our oceans.

For Emeline Bolmont, professor at UNIGE, member of PlaneS and co-author of the study, “this is a complete reversal in the way we look at what has long been called the ‘Faint Young Sun paradox’. It has always been considered as a major obstacle to the appearance of life on Earth!” The argument was that if the Sun’s radiation was much weaker than today, it would have turned the Earth into a ball of ice hostile to life. “But it turns out that for the young, very hot Earth, this weak Sun may have in fact been an unhoped-for opportunity”, continues the researcher.

“Our results are based on theoretical models and are an important building-block in answering the question of the history of Venus”, says study co-author David Ehrenreich, professor in the Department of Astronomy at UNIGE and member of the NCCR PlanetS. “But we will not be able to rule on the matter definitively on our computers. The observations of the three future Venusian space missions will be essential to confirm – or refute – our work.” These prospects delight Emeline Bolmont, for whom “these fascinating questions can be addressed by the new Centre for Life in the Universe, which has just been set up within the UNIGE’s Faculty of Science.”
Yazata Offline
I'm still not totally convinced that dark matter exists.

What we seem to have is excess gravity, given the amount of conventional matter that appears to be present.

So, how can we account for that?

Perhaps there's more conventional matter present than we think there is. I guess that's the so-called 'Macho' theory (massive compact halo objects). I gather that idea is currently out of favor, for reasons that I'm unfamiliar with. But from my layman's perspective, it seems to me to have the parsimonious advantage of not having to invent new kinds of particles, matter or basic physics. Ockham's razor and all that.

Or perhaps our theories of gravity are wrong. I guess that's 'Mond' theory (modified newtonian dynamics). Perhaps gravity doesn't obey the inverse-square law or something like that. This one is kind of a minority idea among astrophysicists at present, perhaps because it's a hugely radical idea. It would throw the rest of astrophysics for a loop if it was true.

And there's today's orthodox dark matter theories, that posit a variety of hypothetical forms of unconventional matter. The 'Wimp' theories (weakly interacting massive particles). But these theories seem to me to invent a lot of very hypothetical new physics as well to account for all the hypothetical unconventional matter. Frankly, I'm not sure why it's the most popular theory at present. (Or at least a theory-schema, since it wouldn't be a fully developed theory until it specifies what the unconventional matter supposedly is.)

From my spectator's seat (munching overpriced popcorn with my feet sticking to the floor) it looks to me like there may be more options still available than just the Wimps.

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