Archive for the ‘SETI’ Category

Dark Matter Stars and SETI

Tuesday, August 5th, 2008

Any species which advances beyond “the Singularity” stage of development has essentially become immortal and is faced with the inconvenient fact that natural power-sources like stars aren’t. While they might persist for eons there will always come a day when the fusion flame goes out and most of a star’s mass is left to cool to ambient temperatures.

But are there other options for powering stars? Two new sources of power have emerged in recent arXiv.org submitted papers, Black Hole Quasi-Stars and Dark Matter Stars. Let’s have a look at those ideas.

Quasistars: Accreting black holes inside massive envelopes

…Begelman, Rossi and Armitage describe a scenario in which an accreting black-hole forms the core of a star-like object. A star is essentially an object in which the pressure created by inward pull of gravity is counteracted by the outward pressure of escaping electromagnetic energy – either indirectly as particle agitation (what we call ‘heat’) or directly as radiation pressure. In side a quasi-star the hot layers of gas above the black hole are bloated into a heat-pressure supported radiating surface, a luminous star, by the energy of infalling matter. As matter falls into a black hole it can lose up to ~5.7% of its mass energy as radiation – this is more efficient than a star’s piddling 0.7% energy production via nuclear fusion.

This might sound rosy as a long-lived star power-source but it has a down-side. It’s violently unstable because of opacity. Opacity is the tendency of a gas to get in the way of radiation passing through it – it’s what turns radiation into ‘heat’. Too much and the radiation gets pent up until something gives… in this case the gaseous envelope that looked so much like a star gets blown away and the naked raging x-ray source that is an accreting black-hole is revealed. Not healthy.

A nicer way to stoke the flames is via dark matter… except we still don’t really know what it is. One candidate is WIMPs – short for Weakly Interacting Massive Particles – and by “Weakly” they interact via the Weak nuclear force as well as gravity. With the least number of assumptions and actual prediction by minimal extensions of the Standard Model (i.e.how known particles and forces work), WIMPs are prime candidates for the super-abundant Dark Matter that out-masses “light matter” 5 to 1. Because they’re Weak force particles, they happily are their own antiparticles and thus a good energy source for a star.

A recent reference…

Dark Stars: Dark Matter in the First Stars leads to a New Phase of Stellar Evolution

…in which the effect of WIMP annihilation on the first stars is discussed. Basically a low mass newly minted star glows brightly from Dark Matter when it gets to about 17 AU across and 0.6 Solar Masses, then it gradually accretes more mass and becomes 800 Solar Mass behemoth radiating 1,000,000 times as bright as the Sun, lives a million years, then dies by collapsing directly into a Black Hole, perhaps forming the core of a Quasi-Star?

Such very early objects allow the super-massive Black Holes at the cores of Galaxies to form rapidly – some eventually massing as much as the Milky Way’s visible matter (~60 Billion Solar Masses.) That doesn’t sound like a very long-lived star candidate, but further work has revealed a light-weight version…

Dark Matter annihilations in Pop III stars

…with the right density of WIMPs (about a billion per millilitre) a 20 Solar Mass star can be ‘frozen’ and still be happily burning on the Main Sequence for as long as the current age of the Universe. That’s a life extension of about ~2,000 fold, so it’s definitely enticing to imagine ETIs shepherding Dark Matter into the Galactic Core and giving their stars a life-extension. With a mass-energy conversion efficiency of ~60% the Galaxy’s 1.2 trillion Solar Masses of Dark Matter could keep its stars burning at 30 billion Solar Luminosities (current output) for ~350 trillion years. Much of that luminosity is from over-active O, B and A stars, so the useful light level is more like ~3 billion, thus 3.5 quadrillion years of starlight is available for all to bask in. Not forever, but substantially better than the darkness awaiting the natural Galaxy in that epoch.

Eventually something better will be needed. But just what will replace starlight?

Another WIMP Star paper is Low mass stellar evolution with WIMP capture and annihilation …which discusses the effect of WIMP burning on low mass stars like the Sun. As already pointed out the stars’ lifetimes are enhanced greatly – after 30 billion years the model star looked as it did at age 0.1 billion. In other words so long as the WIMP supply lasts the structure is stable and nuclear fusion takes a back-seat. The star can last for trillions of years given the right WIMP halo. Gravitationally bound structures composed of more than two roughly equally massed objects are inherently chaotic and prone to eventually fall apart – even the Galaxy will disperse over about 10 million trllion years. Nothing lasts forever.