Order Of Magnitude

In Galaxies by Brian Koberlein9 Comments

Suppose you picked up a single grain of sand and held it at arms length. The sand grain would cover just a tiny patch of sky. Now imagine you found the darkest patch of night sky you could find and looked long and hard at an area no bigger than that single grain of sand. In 2004 the Hubble space telescope did just that, observing a tiny patch of dark sky for a total of 55 hours. The image it produced is known as the Hubble Ultra Deep Field. It found more than 10,000 galaxies that were present when the Universe was just 400 – 800 million years old. 

It was a bit of a shock to find so many galaxies in such a small region. If the HUDF is typical, then there must be about 100 billion galaxies in the observable cosmos. Each galaxy would have about 100 billion stars on average, and stars typically have roughly 10 planets, and hundreds or thousands of asteroids and comets. A single image of sky no larger than a grain of sand showed us the Universe was larger than we’d ever imagined. But it turns out our estimate was wrong by a factor of 10.

New research has looked at the positions and redshifts of galaxies in deep field surveys, and created a 3D map of the distribution of young galaxies. This included many faint galaxies that can be particularly difficult to observe. The map was then compared to computer simulations matching the galaxies by distance and brightness. The simulations indicated that there must be far more galaxies we can’t see than the ones we can. Most galaxies are simply too dim and too distant to be seen with our current telescopes. The vast sea of galaxies seen in the Hubble Ultra Deep Field is just a glimpse of what’s really out there. Rather than a Universe filled with 100 billion galaxies, there are likely 2 trillion galaxies in the observable Universe. That’s about 200 galaxies for every man, woman, and child on Earth.

Imagine if you gave names to 200 galaxies in the cosmos, and so did everyone else on the planet. There would still be billions of nameless galaxies out there. The Universe is vast indeed.

Paper: M. Huertas-Company, et al. Mass assembly and morphological transformations since z ∼ 3 from CANDELS. MNRAS 462 (4): 4495-4516 (2016)
DOI: 10.1093/mnras/stw1866


  1. I never expected that astronomers can do much more than HUDF. It was really great to hear about new reasearch data. I really loved the new image of galaxies.

    Universe is really very large.

  2. If there are 10x the number of previously known galaxies, that would imply that there’s now 10x as much known mass. How does that impact dark matter and dark energy?

  3. If the estimate for the number of galaxies in the universe has gone up by a factor of 20, does this mean that the number of galaxies in that sand grain sized patch of sky is not 10,000 but rather 200,000?

  4. If the same is mass is now to be distributed among 10X the number of galaxies, does that not then mean that estimates of the mass of a galaxy were wrong?

  5. First of all, this post is exactly why the Steady State hypothesis of Hoyle, Bondi and Gold was indeed a testable scientific one. I consider the observation an even more convincing refutation, an ugly fact to slay a beautiful theory (Nobody WANTED semi-infinite Time), than the Cosmic Background radiation.
    If anybody knows exactly why the Big Bang had to be an unimaginably large point-sized mass, rather than an unimaginably dense and four dimensional mass of non-zero dimensions, I’d like to know it. I figured out for myself that a primeval Black TeratoHole (i.e. a more monstrously big one than any other) won’t meet the Ultimate Principle of Relativity, that all observers are equal. A Black Hole has an event horizon, and an observer near the horizon is different from one further inside.

  6. As for what I think I know about Dark Matter and Dark Energy, I’d refer you to Neil deGrasse Tyson on the latter, because he’s admirably brief. “We do not know what’s pushing everything apart, we just call it Dark Energy.” I think I’ve got that right.
    But Dark Matter is easier. The behaviour of whole galaxies, as I understand it, implies a much larger mass performing their mutual attractions than the stuff we can see and deduce from other observations.
    Newton showed that the gravitational field inside a sphere of constant density is the same, at a given radius from the centre, as if from a point mass at the centre equal to the mass of a sphere of that radius
    Now try to figure out the gravitational field of a Ringworld a uniform torus. Anywhere on the surface you’re fine. Presumably exactly on the axis and some distance out, you’d be att
    racted to the dead centre and fly past, then oscillate back again. But from slightly off axis, that oscillation will eventually land or splat you on the torus.

  7. I think we are searching in the wrong way we should think about our mind now we use very little percent of our mind we need to increase it’s percent of use than we will be able to know a lot about every thing

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