Order of Magnitude

Order of Magnitude

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


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