Q: A flurry of blackhole questions!

Q: How much of the universe’s mass is currently in black holes?

Blackholes fall into two basic categories: stellar mass blackholes which have a mass of 3 to 30 Suns (give or take), and super-massive blackholes which usually have masses of more than 100,000 Suns.  Even in our own galaxy it’s essentially impossible to determine whether or not stellar mass blackholes are present.  I mean… they’re black, and they’re not heavy enough to throw around the nearby stars.  However, the supermassive blackholes do throw nearby stars around.  And that star-chucking property has allowed us to find that they have a mass of roughly 0.1% of the “bulge-mass” of the galaxies they sit in (the bulge is just the part of a galaxy that isn’t a disk).  So if I had to make a flying guestimate, I’d say that somewhere around 0.2% of the mass of any given galaxy is tied up in blackholes.

Q: Is there a graph of the number of black holes created since the big bang?

Probably.  Blackholes form from large stars, and large stars tend to have short lifetimes (a mere several million years).  So there should be a pretty sharp correlation between star formation rates and blackhole formation rates.  However, star formation rates are also notoriously difficult to measure.

Q: When was the first black hole created and when will the last one be?

Primordial Blackholes“, if they exist, would have formed almost instantly after the big bang.  If the Big Rip happens, then you can expect the last blackholes to form 50 million years before the end of the universe (give or take).  Otherwise, there’s no telling.

Q: How old will the universe be when black holes start to evaporate, if they even do?

Primordial blackholes should be popping right now.  The lightest stellar-mass blackholes (3 suns) won’t start evaporating until after the universe has cooled to below their Hawking temperature, which should be in about 13 billion years, when the universe is twice as old.  However, one age-of-the-universe is chump change compared to the 1069 years (about 10 billion trillion trillion trillion trillion times the age of the universe) it will take for the first stellar-mass blackholes to completely evaporate.

Q: Could all black holes evaporate away in a expanding cooling  universe?

Yup.

Q: What happens to the universe if all the back holes evaporate away?

No more blackholes?

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10 Responses to Q: A flurry of blackhole questions!

  1. Rod says:

    Thanks – Gives me plenty to ponder at Tea breaks. I love the last answer, I totally deserved that for my cheek to ask the question(s).

  2. christopher says:

    if the gravitational pull of a black hole is so strong even photons cant escape, then how do they evaporate at all?

  3. The Physicist The Physicist says:

    Complicatedly. In a vacuum particle appear and cancel out all the time. If a particle / negative-particle pair were to show up on opposite sides of the event horizon (point-of-no-return), there’s a small chance that the outside particle may escape. It was never quite on the inside of the event horizon, just really really close.

  4. christopher says:

    but then wouldnt the other particle go into the black hole? thus giving the black hole as much energy as it shedded, for a net loss of 0? and if energy cant be created nor destroyed how is that possible anyway?

  5. The Physicist The Physicist says:

    “Negative-particle” as opposed to “anti-particle”. The negative particle has negative energy and mass, so while the ordinary particle carries away energy, the negative half cancels out some of the black hole.
    Before you ask: I don’t know why the negative half always goes inward. No idea at all.

  6. christopher says:

    hmmm, i always thought negative particles were anti particles. now ima have to read on negative particles. negative mass? wtf universe. does that mean it interacts with anti-higgs bosons or there is a negative higgs boson or sumthin?

  7. christopher says:

    this site is awesome by the way, its totally going to hurt my engineering grades tho. you two are causing me to go on wiki-rampages. ironic really. next final ill know all about relativity (well, maybe not ALL :p) but heck if ill be able to draw the load line for a class b push pull amplifier. damn amplifiers. BTW, i dont know if this is just my browser, but every time i post it brings me to a blank page with no back button. so i have to re-look the post up to get back.

  8. The Physicist The Physicist says:

    It’s worth noting that negative particles don’t show up in situations where they get directly measured, only inferred.

  9. The Physicist The Physicist says:

    Kind of you to say!
    We’ll be sure to post crap until after finals.

  10. Jim says:

    In 10^69 years, there won’t be any stars, there won’t be any sources of heat, and there won’t be any life. But at least it will be really, really big!

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