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Cake day: January 2nd, 2024

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  • PBH with mass <10^6g would have evaporated before the universe had cooled enough for atoms to form. Its possible they didn’t fully evaporate, but instead became “Plank relics”, which are a dark matter candidate.

    PBH with mass 10^7g to 10^16g would have evaporated already, producing a background of gamma rays and gravity waves that we don’t see.

    PBH with mass 10^17g to 10^22g would still exist today, and the gravity waves they generate are too small to be detected by current detectors. These are also a dark matter candidate.

    PBH with mass >10^23, in sufficient numbers to explain the existence of dark matter, would cause gravity lensing that we don’t observe.

    So according to observations, if the early universe produced PBH, they didn’t have an even distribution of masses from giant to tiny. Either they were all tiny (<1 ton), or they were all medium size (asteroid mass).

    My favorite explanation of dark matter is the formation of asteroid mass PBHs when the early universe went through the phase change that separated the electroweak force into the electromagnetic force & nuclear weak force. Just a bit before electroweak symmetry breaking, the universe was in a state of supercooled false vacuum, and then bubbles of today’s vacuum energy started expanding. The pockets of false vacuum between the expanding bubbles of true vacuum would be slower to inflate, causing their density to grow relative to the rest of the universe, until they collapse into PBH. Because they’re all formed at the same time, from similar size pockets of similar density plasma, the resulting population of PBH are uniformly asteroid mass rather than having a Gaussian mass distribution.

    Further reading:

    Gaussian Planck Relics are Ruled-Out as Dark Matter by LIGO

    Constraints on primordial black holes from the Galactic gamma-ray background








  • The paper linked to in the article says the thrusters have a specific impulse of 600s, and a thrust-to-power ratio of about 50 mN/kW.

    Compared to the xenon ion thrusters used on the Dawn spacecraft, these new multi-mode thrusters produce more thrust, but are significantly less efficient. Dawn’s thrusters have a specific impulse of 3,100s and a thrust-to-power ratio of about 36 mN/kW.

    Even so, it means satellites can be built with one small fuel tank that can power high efficiency electrospray thrusters to make slow maneuvers, or use the same fuel as a monopropellant to quickly get out of (or into) the way of something. ASCENT monopropellant thrusters can have a specific impulse slightly better (240 Isp) than traditional hydrazine monopropellant thrusters (235 Isp).