Astronomy - September 2015

Our

+++= universe Yo u

Electromagnetic

force

Other universes

Strong

nuclear

force

Weak

nuclear

force Gravity

Big Bang

Inflation

Time

Time

Time

Yo u

Yo u

Yo u

ASTRONOMY

: ROEN KELLY, AFTER BICEP2

32 ASTRONOMY • SEPTEMBER 2015

Math makes it real

Then you have the mathematical universe hypothesis, also

called the ultimate ensemble. This multiverse contains

everything that is mathematically possible. It’s cham-

pioned by those, like Tegmark, who think mathematics

is the basic reality of the cosmos as opposed to being a

human conceptual tool. This view says that everything

that can happen mathematically does happen in its own

separate universe.

The anthropic principle provides more multiverse

rationales. This is the theory that while our universe is

simple in many ways, it contains dozens of physical con-

stants whose values are Goldilocks-perfect for life to exist.

If the fine-structure constant that governs the strength of

the electromagnetic force was barely different, or the

power of the strong force or the strength of gravity were

slightly tweaked, there could be no atoms, no stars, and no

chance for life. Ours is a universe so precisely fine-tuned

for life, science must ask why.

By arguing for the existence of countless other random

universes, the vast majority of which would not have the

physical properties that permit life, multiverse advocates

can say, “See? Taken as an aggregate, nothing special is

going on. The multiverse landscape contains every sort of

universe, and we just happen to live within one of those

that supports life.” It is a way to make our exceptional-

seeming cosmos less extraordinary.

A few other multiverse candidates exist, but you get the

idea. The whole thing is exciting because it unveils breath-

taking new possibilities that paint the overall universe to

be even vaster than we’d assumed.

The infinite yous

Speaking of “vast,” let’s go back to that infin-

ity business, which figures prominently in

multiverse models. If true, there’s definitely

another “you” out there on another Earth in

a nearly identical universe to ours. After all,

infinity means no limits. Guth maintains that

a multiverse leads to the conclusion that “any-

thing that can happen will happen — and it

will happen an infinite number of times.”

So another “you” exists who wore the same

socks to the prom with the little hole in one.

This other you even has your cat, with identi-

cal markings. Infinity means there wouldn’t

be just one exact copy of you with matching

dental fillings. No, there are infinite duplicate yous. There

also must be an endless army of yous with one opposite

personality trait. None of them would ever leave a dirty

glass in the sink.

Doesn’t smell right? Happily, there’s been a recent anti-

infinity trend among physicists. Last year, a prominent

pair of theorists, mathematician George Ellis and physicist

Joseph Silk, wrote an editorial in the journal Nature

encouraging cosmologists to heed the warnings of math-

ematician David Hilbert, who died in 1943. “Although

infinity is needed in mathematics, it occurs nowhere in the

physical universe,” the pair opined.

In any case, internal mathematical consistency is not

enough to adequately support a multiverse theory, whether

it involves strings, inflation, or hypothesized higher-

dimensional colliding membranes that could smack our

universe into another. That’s because math can diverge

from physical reality, as Zeno showed in 450 b.c. with his

famous Achilles and the tortoise thought experiment.

But multiverse hypotheses generate deep objections that

go beyond their underlying math. Critics say they are non-

falsifiable and thus indistinguishable from philosophy and

no more useful than saying, “God did it.” Last December,

Ellis and Silk wrote, “The imprimatur of science should be

awarded only to a theory that is testable.”

Going further, last year, writing in Edge, Paul Stein-

hardt, Princeton University’s Albert Einstein Professor in

Science and an early inf lation advocate, wrote, “The notion

that we live in a multiverse in which the laws of physics

and the properties of the cosmos vary randomly from one

patch of space to another [should be retired].”

Strong words. Well, should we really ax the whole mul-

tiverse business? A big problem many critics cite is that

multiverse models don’t predict anything, and thus they

allow everything. Moreover, Steinhardt is convinced mul-

tiverse theories are put forward mainly to try to salvage

failed hypotheses like string theory.

PERFECT EQUATION

MATH MODEL

INFLATION

THEORY

INFINITY

MODEL

Astronomers thought this

swirl pattern in the cosmic

microwave background

was caused by gravitational

waves from the early uni-

verse’s rapid expansion due

to inflation. But follow-up

observations showed it was

mostly from galactic dust.