Last year, researchers found one of the farthest-ever galaxies. This smudge on a Hubble photo is said to sit at a distance of 13 billion light-years (abbreviated 13 Gly). It was born soon after the Big Bang and has been given the catchy name A1689-zD1.
But viewing galaxies whose light traveled a long time through an expanding universe introduces several bizarre twists in understanding. Let’s finally get this straight, once and for all.
Nothing is particularly screwy in our own close-up neighborhood of space, at least outside Washington. The galaxy M87 in the Virgo cluster lies some 50 million light-years away. So its light took that long to get here. That’s short compared with its age of at least 8 billion years; its image is therefore a current, accurate snapshot. Also, the distance from here to M87 grows by 800 miles (1,300 kilometers) every second. This redshifts (stretches) its light, which makes the galaxy slightly dimmer. The effect is barely noticeable because M87’s recession is less than 1 percent of light-speed.
Even if we jump 20 times farther and observe any of the 60 million galaxies that float within 1 billion light-years of Coney Island, we see them essentially where they really are and how they look today.
But now the fun begins. Because the speed of the expanding universe in-creases with distance, weird stuff happens to truly faraway objects. Take A1689-zD1. We can say it’s old because we see the galaxy as it was when its light started traveling to us 13 billion years ago. The galaxy’s image is ancient. We can also say it’s young because we’re viewing a picture of a newborn object.
But is this galaxy really 13 Gly away, as the news articles claim? Cosmologists use cool calculators to answer this sort of thing. When the image we’re seeing left A1689-zD1, we were much closer to-gether. It was then only 3.35 Gly from us. So the image should logically display the size of a galaxy at that nearer distance, when its light left, rather than the distance the light had to cover in order to get here. The image’s dimensions don’t change just because it took a long time to reach us.
Amazingly, that galaxy does look larger than we’d expect for something so far away. It measures 1/67 the angular size of nearby Virgo galaxies of the same type. If it were 13 Gly away, it would appear 1/260 the size of Virgo galaxies. Holy cow! It’s like a funhouse mirror. The galaxy appears much closer than it is!
Let’s put all this together. It’s the oldest galaxy image we’ve ever seen, which also makes it the youngest. It looks way too big for its distance, but also way too faint. Could things get any weirder?
You bet. Science articles say it’s 13 Gly from here because distance is often ex-pressed that way. But that’s merely how long its light took to reach us. During all that time, A1689-zD1 has been madly receding. This galaxy is now actually 30 Gly away. To get the true figures for these kinds of questions, use the calculator cosmologist Ned Wright of UCLA posts. Punch in a light-distance, and it’ll give you the rest: http://www.astro.ucla.edu/~wright/DlttCalc.html
Now consider cosmic boundaries. The first stars or protogalaxies may have formed as early as 100 million years after the Big Bang; it’s still debated. If so, they’ll display angular sizes as if they’re just 1.2 Gly away. But their brightnesses would indicate an inconceivable distance of 1.2 trillion light-years — utterly undetectable. Their actual distance today would be 38 Gly. These parameters roughly mark the edges of the observable universe.
But objects do not end there. Most galaxies were never positioned for their light to be able to arrive here at all. Astronomers such as Wright believe at least 98.4 percent of the universe lies “over the horizon” in a zone that can never be observed. This can be a frustrating limitation. For example, something massive is disrupting the universe’s smooth expansion in our region of space. Some sort of “great attractor” lies in the direction of Centaurus. Many astronomers think it’s located outside our observable reality. It tugs on our visible universe from a place beyond.
That “place beyond” constitutes nearly all of the cosmos. And where does that end? No one’s sure, but it’s likely the universe is, was, and always will be infinite in extent. If this is true, everything we can ever see represents 0 percent of the total galactic inventory.
July 2009: Facing reality
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