“Birth and death; we all move between these two unknowns.” -Bryant H. McGill
One of the most remarkable consequences of the Big Bang is that the Universe as we know it — full of planets, stars, galaxies and life — hasn’t been around forever! Because the Universe is expanding and cooling, it was hotter, denser, and more compact in the past.
But these things that fill the Universe today weren’t there in the very early stages of the Universe; they took time.
Gravity needed many millions of years to collapse these slightly overdense regions to a point where even the first stars could be formed, much less galaxies and clusters of galaxies. Well, if you’re curious, you might ask this question about black holes: was the Universe born with them, or did they, too, need to wait for millions of years for gravity to do its thing?
Some of the most remarkable things that our Universe contains are black holes, which come in two known varieties. On the one hand, there are the kind that are around the mass of our Sun, that come from the deaths of massive stars.
And of course, these guys took at least as long to make as it took to form the massive stars. We have a huge number of black hole candidates in our own galaxy, shown in red — superimposed over our galaxy’s stars — in the image below.
But there are also much more massive black holes. These range from black holes thousands of times the mass of our Sun (in purple, above) to the supermassive ones (in blue) found at the centers of galaxies. The supermassive ones, in fact, can be millions of times the mass of our Sun — like in our own galaxy — up to several billion times that mass, like in the galaxy M87. Regardless of size or activity level, every large galaxy we know of has a supermassive black hole at its center.
These two types are generally accepted by the scientific community, but there is another type that occasionally gets some attention as well: miniature black holes! Smaller in mass than single stars but larger than about the mass of a mountain (otherwise they would have evaporated by now), these rogue black holes could be strewn throughout the Universe, including our galaxy, and are sometimes speculated to be a significant fraction of dark matter.
This idea was interesting when it was developed in the 1970s and 1980s, but we know a whole lot more about the Universe now than we did then. Let’s ask what we need to form these black holes back when the Universe was first born, and see if we can determine how much validity there is to such an idea.
If you want to gravitationally collapse the matter in any region of space — which you need to do if you want to make anything, including a black hole — it needs to have a great enough energy density to overcome the expansion of the Universe. According to our understanding of structure formation, runaway collapse — what specialists call non-linear structure formation — happens once your energy density in one region of space becomes about 60% greater than average.
So if you want your Universe to be born with black holes, you need some small regions of space to be born with about 60% more energy than average. Let’s take a look at the Universe and see what it’s doing when it’s very young.
This image shows the fluctuations, or the departure from average temperature, in the leftover glow from the Big Bang. The average temperature — as we see it today — is 2.725 Kelvin. So, you’d reason, if you had warm regions where the temperature was somewhere around 4 Kelvin, and the cool regions were somewhere around 1 Kelvin, you would form these black holes in very short order after the Big Bang.
This wouldn’t happen on large (i.e., degree-sized) scales, though; they would have to have these fluctuations on much smaller scales. Let’s take a look at how these fluctuations, from when the Universe is 380,000 years old (when the leftover glow from the Big Bang is emitted), change with scale.
As you can see, these fluctuations are nowhere close to being of Kelvin scale; we’re talking tens to hundreds of microKelvins, at most. What’s even worse is if you remember that we need to have larger magnitude temperature fluctuations on smaller scales to get these micro-black-holes.
And as both a simple visual inspection and a detailed analysis of the scalar spectral index show, the initial fluctuations are not larger on smaller scales.
So while it might be fun to speculate about small, primordial black holes, to the best of our knowledge they are not part of our actual, physical Universe.
Much like Vince Lombardi once said about leaders, the same is true of black holes, that they
aren’t born, they are made. And they are made just like anything else, through hard work. And that’s the price we’ll have to pay to achieve that goal, or any goal.
Gravity will do that work for you, in the end. But you have to give it enough time to do its job. In the beginning? The fluctuations are simply too small. But given millions of years, they’ll grow, collapse, form stars, and then you’ll get your black holes.
Massive, very massive, and supermassive black holes should all exist. But the tiny ones?
Not in our Universe.