## How do we observe galaxies further than 13.7 billion light years away?

z8_GND_5296. Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA

Sorry for such a lengthy title, but the subject came up because of a widely circulated announcement of the discovery of “The Most Distant Galaxy Yet Seen,” a title that’s being constantly revived (Matt Francis over at Universe Today has a nice discussion of why this galaxy really is a big deal).

The galaxy, which for now goes by z8_GND_5296 was discovered by the CANDELS collaboration at a distance of about 30 billion light-years.

“But wait!” my friends said on facebook and twitter, “how can we even see a galaxy 30 billion light years away when the universe is only 13.7 billion years old? Isn’t light the ultimate speed limit in the universe?”

Yes. It is. But the universe was smaller in the past.

What does “size of the universe” mean?

To begin with, the CANDELS team didn’t actually find that the galaxy was 30 billion light-years away directly. Rather, they found that it had a “redshift” of about 7.5. Or, to put it another way, the universe was about
$a_{emitted}=frac{1}{1+z}=frac{1}{1+7.5}=frac{1}{8.5}=11.7%$
the size it is now, and we measure that fact by noting that light that left this distant galaxy has grown by a factor of 8.5 in the time it takes to reach us. That’s what redshift is all about. Long wavelengths of light are “redder” than blue ones.

Now for the misconception. Naively you might suppose that if the universe were 11.7% the size it is now at some point in the past, that must mean that the bounds of the universe were 11.7% smaller than they are now. But the universe has no bounds!

Instead, the standard picture is that the universe is much like a balloon, and as it inflates, the distances between galaxies becomes larger by a fixed rate. “Doubling in size” is shorthand for “galaxies getting twice as far apart from one another,” as well as “gas and dark matter becoming eight times as diffuse.” (8 times, because the universe increases in each of 3 dimensions). Here’s a particularly crude version of the whole shebang that Jeff drew for the User’s Guide:

Light and Space

Imagine that, rather than beaming light to us directly, our pal z8_GND_5296 sent us a signal by means of post-stations or whisper down the lane. It sent a signal to a galaxy a few million light years away from it. Galaxy 1 sent a signal to galaxy 2, and so on, until we got the message. Further, imagine that each of those galaxies are currently 10 million light-years apart from one another. This, by the way, is known as their “comoving distance” if you want the technical term.

But the first signal took much less than 10 million years to transmit. Because the universe was so much smaller then than now, it took about 1.1 million years. The next 10 million light-years of comoving distance might be traversed by a light beam in 1.3 million years, and so on, so that after 13 billion years of whispering down the lane, the total distance now between us and z8_GND_5296 is about 30 billion light years.

I warn you, though, even this 30 billion light year number isn’t terribly useful. After all, if I say that it’s 2 miles to the drug store, the implication is that you’ll have to traverse 2 miles to get there. On the other hand, a galaxy 30 billion light years away will be significantly farther by the time you try to reach it. Indeed, there are galaxies out there that, even traveling at the speed of light, we couldn’t ever reach. That’s just a consequence of living in an expanding universe.

What is the maximum distance?

Though we can see to a distance of more than 13.8 Billion light-years, we can’t see infinitely far. The ultimate limit is what’s known as the particle horizon, which for us is a (comoving) distance of about 45 billion light-years. Anything further than that and we have no hope of seeing it.

On the other hand, the universe also has an “event horizon” (yes, just like a black hole), the maximum distance we ever could reach. Because our universe is accelerating, the limit maxes out at a certain point, and there are regions of space that are forever inaccessible to us. Those are systems more than about 60 billion light years from here.

Beyond that is anyone’s guess.

-Dave

ps If there’s interest, I may do a followup on this as to why it is that cosmologists claim there must be cosmic inflation. It’ll even use Zeno’s paradox to talk about how the smallness of the early universe was trumped by its youngness.

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## Blastoff!

Good news, everyone! I was on the Professor Blastoff Podcast this week talking about “The Universe” (which may win the prize for best episode name) with Tig, David and Kyle. We talk about time travel, symmetry, antimatter (and Uncley matter, a joke that I failed to get for about 10 minutes), and much more.

Check it out, and don’t forget to subscribe via iTunes. Oh, and a warning for the faint of heart. There’s a fair bit of saucy language.

-Dave

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## Higgs!

Francois Englert and Peter Higgs

In a move that surprised almost no one in the physics community, the Nobel Prize in Physics was awarded this morning to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”

This is a huge deal, and very well deserved. I won’t go on and on about the Higgs right now, in large part because I’ve already written a ton on the subject, including:

Also, of course, there’s a whole chapter about the Higgs and the Standard Model in my new book, The Universe in the Rearview Mirror.

Congratulations again to Drs. Englert and Higgs!

-Dave

## What do you really need to know about the universe?

Just a quicky. I have a new, somewhat navel-gazing post up at io9 on what you really need to know about the universe, and more generally, why people embrace pop-sci at all. I’m curious to hear what you think, and of course, I’m always looking for more questions for the column.

-Dave

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## Talks at Google

Hi. Would you like to see me give a talk on symmetry? Of course you would. Here’s a professionally produced video of my presentation to the good folks at Google entitled, “Why Symmetry Matters.” Enjoy!

-Dave

ps I also had a really nice visit from a former student who’s now off to grad school. She made me a little Thank You gift that just about made me cry.

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## Slate: 4 Reasons you Shouldn't Exist

Greetings, true believers. I have a piece up at Slate today: Four Reasons You Shouldn’t Exist: Physics says you’re an impurity in an otherwise beautiful universe.. The headline might be a bit over the top, but it’s a fun little piece about the importance of symmetry breaking. Take a look, and then when you’re done, be sure to check out a piece I did a few years ago: A Physicist Looks at the Time-Traveler’s Wife.

-Dave

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## New York, New York

I’m in the Big Apple! And I’m pleased to say that, having made it here, I’m confident that I can make it anywhere.

Today I was on the Leonard Lopate Show for a pretty long interview. We talked about symmetry, my book, a bit about time travel, and finished it off with a nice chat about Emmy Noether. Do check it out. I think it went pretty well.

I also checked out the Union Square B&N to make sure they prominently placed my book (they did), saw some friends, and had some delicious brooklyn ice cream.

Tomorrow, I’ll be giving a google talk at the NY HQ. The YouTube video will be linked when it’s up.

Thanks for your continued support!

-Dave

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## Welcome, Wired visitors!

Hello there.

If you happened to find yourself clicking in from Wired, let me welcome you to my blog. There are some interesting things here (including slightly more technical versions of some pieces from io9, observations about the publishing industry, and lots and lots of updates about my new book. But if you’re looking for something a bit more specific:

1. Go here to read endorsements and reviews of “The Universe in the Rearview Mirror,” including a very nice one from Sean Carroll, with whom I had my Wired dialogue.
2. Go here to check out my io9 “Ask a Physicist” columns. They’re some of my better stuff on the web. Way better than the blog.
3. Go here to follow me on facebook and find out about talks and the like.
4. Go here if you’d like links, comments on physics, and occasional silly observations, limited to 140 characters.

And to my regular readers who’d like to know about what this Wired chat is all about. Sean and I had an exchange on what the public needs to know about science (if anything) and what obligations science popularizers (such as ourselves) have to the public. It was a lot of fun, and you should check it out.

-Dave

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## Two weeks in

What a couple of weeks! In the last two weeks I’ve:

• Done about 20 radio interviews, with several more on the way (WAMC). I’m doing WHYY’s NewsWorks Tonight in about a week, and WNYC’s Leonard Lopate Show on August 20. I also had a really nice interview on philly.com.
• Had lots and lots of great press. Universe Today said:

At most times in Rearview Mirror, Goldberg’s style feels more like a discussion than a book – it’s as if your delightfully nerdy friend from college (the one with a knack for identifying stars, he’s convinced it’s a total turn-on) came over for dinner one night to talk about his favorite topic – the mysteries of the cosmos.

There’s also a major review coming out in New Scientist next week. It’s generally quite positive with a few criticisms (some valid, a few not, but it’s British, so you have to correct for that), with a very strong finish:

Fortunately, good sense, humour and inventive explanationsmanage, like matter, to triumph over their evil twins. Goldberg has a fine ear for the absurd, and is deft at revealing why things we take for granted, such as the equality of gravitational and inertial mass, are strange and not obvious at all.

In the end, Goldberg has a good stab at fulfilling his ambitions. His enthusiasm may sink into scattiness at times but it carries you on a ride through picturesque bits of science. We zoom by Maxwell’s demon, quantum teleporters and black holes, then plunge down for a glimpse of the secret underground, where physics and reality rest on foundations of pure symmetry.

It pains me to say it this way, but it’s a bit like a kick-ass rollercoaster built through Dwarrowdelf in Middle-earth.

(where the last probably requires some context).

• Sold lots of books! We were the #1 Science book at Barnes and Noble last week, at least according to my editor! Woot! Two weeks in and we’re going to a second printing!

Thanks to everyone for giving this book such an enthusiastic reception. Please continue to tell your friends, review online, and on and on.

-Dave

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## Pub Day!

Hurray! The day has finally come. If you haven’t already bought up your copy what are you waiting for?

Seriously, though, thanks to everyone who helped with the book (and to the many, many io9 readers who’ve asked questions over the years). As a gesture, here are my formal acknowledgements from the book:

Writing a book is fun, but you know what? It’s also hard work, and I wouldn’t have been able to get through it without the love, support, and feedback of a lot of people. First, and foremost, my wife, Emily Joy, has read every line and has given me the tough love that I so desperately need. I thank my family, and (though they didn’t give me any comments at all) my daughters, Willa and Lily.

Thanks to all of my friends and colleagues who provided comments and feedback on various parts of the draft and useful conversations: Sean Carroll, Rich Gott, Richard Henretta, Andy Hicks, Lynn Hoffman, Josh Kamensky, Adrienne Leonard, Sean Lynch, Kate Mason, Kevin Owens, John Peacock, Dawn Peterson, Tina Peters, Som Tyagi, Liz Fekete Trubey, and Enrico Vesperini.

I also want to acknowledge Annalee Newitz, Charlie Jane Anders, and everyone at io9.com. Annalee and Charlie Jane were very supportive throughout, and many of these topics were road tested at io9. I would also like to thank all of the readers whose comments helped identify roadblocks to comprehension (theirs and mine).

Not every academic environment smiles on popular science writing as a serious pursuit. I’d like to thank my department head, Michel Vallieres, and my dean, Donna Murasko, for taking the larger view of scholarly work.

I’d like to thank my agent, Andrew Stuart. Andrew did more than simply sell this book; he was an excellent sounding board and advocate and was instrumental in helping me find my voice. My editor, Stephen Morrow, and his outstanding assistant, Stephanie Hitchcock, were tough but fair with their virtual red pen and in stemming my natural verbosity (and trimming some of the more terrible jokes) they improved the book immensely. Thanks also to my amazing illustrator, Herb Thornby, who so deftly and beautifully conveyed ideas that my words may have otherwise obfuscated.

I also gratefully acknowledge Michael Blanton and the Sloan Digital Survey Collaboration for use of their data.

Finally, here are some fun places to go on the web and in real life today:

• At 2pm EDT (though you can start early), I’ll be doing a Reddit “Ask me Anything.” Please do.
• At 7pm EDT, I’ll be doing a reading and signing at the Barnes and Noble in Rittenhouse Square. Please come.
• Some very nice book blogs have had some very nice things to say about the Universe in the Rearview Mirror, including:

Goldberg is a simply wonderful guide to and through all the thickets of complexity and obscurity that one encounters along the road of trying to understand what makes everything tick.