'The Walking Dead's' brutal season ends

Rick Grimes, played by Andrew Lincoln, center, tries to keep some sense of normalcy in a post-apocalyptic world overrun with walkers on AMC's "The Walking Dead," but characters come and go quickly on the gruesome show. Here's a look at some of the key characters we've lost:

Among the most shocking deaths this season was that of young Lizzie (Brighton Sharbino, left), who died at the hands of Carol (Melissa Suzanne McBride, right) after killing her younger sister Mika. Lizzie's sense of right and wrong and life and death had been warped by the zombie apocalypse. Just as she did earlier in the season, Carol had to make the decision that someone was too dangerous to live. Carol urged Lizzie to follow her therapy of "look at the flowers" before she shot her in the back of the head.

Shane Walsh (Jon Bernthal) turned on his best friend, Rick Grimes, and lured him into the woods, apparently with plans to kill him and steal Rick's wife, Lori. Rick stabbed Shane, but he came back as a zombie, and Rick's son Carl had to shoot Shane to put him down.

T-Dog (IronE Singleton) was bitten helping Carol escape zombies at the prison.

Dale Horvath (Jeffrey DeMunn) was killed by a zombie.

Lori Grimes (Sarah Wayne Callies) died during childbirth. Her son, Carl, apparently shot her (heard, but not seen on camera) to prevent her from becoming a zombie.

Sophia Peletier (Madison Lintz) got lost in the woods. She later turned up as a zombie locked in the barn on Hershel Greene's farm. Sheriff Rick Grimes shot her in the head to finish her off.

Amy (Emma Bell) was bitten by a zombie. Her sister, Andrea, had to put her down after she revived as a zombie.

Merle Dixon (Michael Rooker) was killed by the Governor, reanimated as a zombie and put down by his brother, Daryl, who stabbed him multiple times.

Andrea (Laurie Holden) was bitten by Milton, who became a zombie after being stabbed by the Governor. She shot herself so she wouldn't become a zombie. This was heard off camera but not seen.

In one of the show's most heartbreaking losses, Hershel Greene (Scott Wilson) was decapitated and killed by the Governor.

After being stabbed by Michonne, the Governor (David Morrissey) was shot to death by Lilly.

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• "The Walking Dead's" fourth-season finale airs Sunday night


• As Carol, Melissa McBride has been at the center of some brutal scenes this season


• The actress promises a "crazy" finale

(CNN) -- Season four of "The Walking Dead" has featured some of the most brutal scenes in the popular series' history. (Yes, that's a warning for those behind on the show. Spoilers ahead!)

There was the gut-wrenching death of Hershel in the midseason finale, but there was also the burning death of David and Karen early on. It was a mystery who killed them, but it soon came out that Carol (Melissa McBride) did it to stop them from spreading a deadly disease through the prison.

When Rick learned what happened, he banished Carol from the prison, but she reappeared in the second half of the season.

Young sisters Lizzie and Mika were left under Carol's care after their father died, but it became clear that Lizzie had a skewed sense of right and wrong, and she killed her younger sister to bring her back as a walker.

Then Carol decided yet again that another survivor was too dangerous to live and shot Lizzie in the back of the head.

The reaction to this episode was huge. With McBride at the center of some of this season's more memorable scenes, CNN spoke to her about Carol's evolution and the finale Sunday night.

CNN: What was your initial reaction when you learned what Carol was going to have to do this season?

McBride: It was surprising to me that Carol was going in this direction, absolutely. But it also made a great deal of sense to me that they would take her in this direction. First of all it's a very interesting arc for this character, considering that she's lost her daughter; she carries so much responsibility for that, and also (she's) trying to navigate this world.

CNN: Is this the most challenging acting role you've ever had?

McBride: By far. This has been by far the most difficult, and certainly the longest-running. The most challenging and rewarding, for all the same reasons.

CNN: There was a powerful response online to the deaths of Lizzie and Mika; did you follow that reaction at all?

McBride: Yeah, I was really happy about the responses that came from that episode. Shooting that episode was remarkable and such a great experience. It all just came together in such a wonderful way. People talk about how "it needs to be earned." I felt what was necessary in this episode was absolutely earned. The emotional and critical response has been wonderful. The fans have just been phenomenal in their appreciation for that episode. What ultimately happened in that episode was controversial, but I feel like it was handled with such a sensitive approach. The build-up to it was just handled so well.

CNN: Have you seen the memes that have cropped up about the scene? The line "Look at the flowers" has taken on a life of its own.

McBride: People are pasting people's faces over Lizzie's in the flower bed! (Laughs)

That's another thing I love about the fanbase: They have a great sense of humor. ... I'm a spoofy kind of person. I appreciate the humor, and it's a great way to offset the devastation that people on the show (are) going through -- it's sometimes very refreshing to see that stuff.

CNN: There's a segment of fans who like the idea of Daryl and Carol together. They haven't had too many scenes together this season, but is that something you might be pulling for?

McBride: I know that Carol and Daryl have a tremendous bond. Personally, I am living vicariously through the fans, I guess. I enjoy their passion. For some people, watching the show, it's a fun element for them. They're very passionate about their 'ships!

CNN: How do you react to the show's massive popularity?

McBride: I'm constantly amazed by the amount of different types of people that are aware of the show. After the episode with the girls, a gentleman who is 73 years old -- he said, "I've never written a piece of fan mail ever, but I had to write how much I enjoyed that episode." It touched me so much.

When I think of how far-reaching this show is, and how big it is, it's like trying to fathom the beginning of time, and I have to stop.

CNN: What's your favorite thing coming out of the show?

McBride: It certainly has to be the relationships within the cast and crew, and getting to work with the people I do. It's so tremendously fulfilling, and personally getting to play this character has been a dream.

CNN: What can fans expect from the season finale?

McBride: There's nothing I can say that hasn't already been said! Like every finale, (whispers) it's gonna be crazy!

Greene: Want a job? Deliver, or protect

Life.com pays tribute to mail carriers, other postal workers and the U.S. Postal Service in earlier times. Here, a rural Vermont mail carrier makes his rounds in subzero weather.

A 24-hour automated post office in Maryland provides self-service in 1964.

In New York in 1944, Christmas packages intended for American servicemen who have been listed as missing or killed in action await "return to sender" stamps.

A mail carrier stops on the sidewalk in Michigan in 1955.

A peek inside a Washington post office in 1942.

• Bob Greene: America may end up with only two dominant occupations


• Greene: One is carrying stuff to people's homes, the other is protecting them


• He says with the rise of online shopping, more people need delivery services


• Greene: The other growth field is private security; we employ more guards now

Editor's note: CNN Contributor Bob Greene is a bestselling author whose 25 books include "Late Edition: A Love Story," "Duty: A Father, His Son, and the Man Who Won the War," and "Once Upon a Town: The Miracle of the North Platte Canteen," which has been named the One Book, One Nebraska statewide reading selection for 2014. The opinions expressed in this commentary are solely those of the author.

(CNN) -- If the trajectory of American daily life keeps heading in its current direction, the country may end up with only two dominant occupations:

-- Carrying stuff in boxes to people's houses.

-- And keeping a watchful eye on those same people so they don't hurt each other or steal things when they do leave their homes.

For all the talk about how e-commerce is outpacing the performance of traditional retail sales in brick-and-mortar stores, the men and women who could gain the most from the boom in online merchandising are not Web designers or those who come up with clever new shopping apps, but workers who perform one of the oldest jobs in the world:

Lugging packages from the curb to someone's front door.

It's one task that can't be accomplished in cyberspace. During the 2013 holiday season, according to the Wall Street Journal, retail stores drew only about half of the foot traffic that they did three years earlier. And during the peak holiday shopping period, online sales increased by more than double the rate of sales at traditional stores.

For every shopper who doesn't leave a store carrying a bag full of merchandise, but instead places orders online, there's someone who has to bring those items to the residence. And in the long run, the major beneficiaries of this shift in habits could very well turn out to be the people who undertake a decidedly low-technology endeavor: grabbing a box from the back of a truck and walking with it to a house.

You can already see the effects of this in statistics kept by the United States Postal Service. It's no secret that, in our era of e-mails and texts, the number of first-class letters delivered by the post office has plummeted. As recently as 2008, the Postal Service reported that it was delivering 90.7 billion letters annually. By last year, that number was down to 65.8 billion.

But the Postal Service's numbers for delivering packages, as opposed to letters, have actually gone up. In 2008, it delivered 3.3. billion packages; that had grown to 3.7 billion packages by last year. United Parcel Service says it delivers 16.3 million packages and documents worldwide every day -- 4.1 billion a year.

Some time-honored, pre-digital-era manufacturing firms have figured out a way to make the lug-the-package-to-the-house business model work for them, too. International Paper Co., according to the Journal, "is among the companies profiting from new digital habits. It bought several makers of corrugated cardboard boxes, which now fill with goods shipped by online retailers like Amazon. The Memphis-based paper company said it has a 35 percent market share."

But if carrying boxes is one line of work that has legs, another -- just as old-line -- may become even more in demand as the years go by. We can tell ourselves that our worlds are full of social media friends and amiable followers, yet the growth in industries devoted to protecting us from each other, both on the streets and in the electronic ether, betrays a certain distrust in the purportedly benign intentions of our fellow citizens.

It's not just the checkpoints at airports, and surveillance cameras on street corners, and cybercrime-fighting software offered by manufacturers; Samuel Bowles and Arjun Jayadev, writing in the New York Times, noted, "another dubious first for America: We now employ as many private security guards as high school teachers -- over one million of them, or nearly double their number in 1980. ... What is happening in America today is both unprecedented in our history, and virtually unique among Western democratic nations. The share of our labor force devoted to guard labor has risen fivefold since 1890."

This caution on the part of both citizens and industry may be warranted, but Bowles and Jayadev, reflecting on the melancholy underlying message of this, point out what philosopher John Stuart Mill wrote in 1848: "It is lamentable to think how a great proportion of all efforts and talents in the world are employed in merely neutralizing one another."

Neutralizing one another or, more cheerily, showing up bearing boxes. The Postal Service reports that its letter carriers and package-deliverers drive 4 million miles every day. UPS operates a delivery fleet consisting of 96,028 cars, vans, tractors and motorcycles, as well as 237 aircraft. As the inexorable shift to e-commerce becomes the standard way of shopping, those numbers can only be expected to increase.

And the private-security industry? That other growth field? There is an outside chance that it could eventually dwindle: if we should someday decide to put our faith in the sweet intentions and endless goodwill of our fellow citizens, both the ones we can see and the ones who invisibly make their way into our computers.

Until then, though, there is a multitude of home-security and break-in-alarm products for sale online.

And a multitude of men and women hired to carry them to the double-locked front door.

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LZ: Christian silence on gay hate

• Pope Francis and President Obama discussed the importance of empathy


• LZ Granderson says the Pope was right last year when he said: Who am I to judge?


• Sadly, he says, many others don't follow that teaching


• Granderson: Don't answer the persecution of gay people with silence

Editor's note: LZ Granderson is a CNN contributor, a senior writer for ESPN and lecturer at Northwestern University. Commentary by the former Hechinger Institute fellow has been recognized by the Online News Association, the National Association of Black Journalists and the National Lesbian and Gay Journalists Association. Follow him on Twitter @locs_n_laughs. The opinions expressed in this commentary are solely those of the author.

(CNN) -- Pope Francis sent shockwaves around the globe last July when in addressing the issue of gays in the church, he opted not to remind his 1.6 billion followers about the fiery pits of hell but instead posed a question: who am I to judge?

Not exactly a ringing endorsement for equality but the rhetorical question was light years away from what we have grown accustomed to hearing from his predecessors. And because of that, we felt a glimmer of hope that maybe the needle within the larger Christian community had moved from hostile rhetoric to civil engagement.

Maybe even tolerance.

In their nearly one-hour meeting this week, Pope Francis and President Barack Obama reportedly spent the bulk of their time discussing empathy.

Community rallies around gay student

Catholic: 'We're being besieged'

Ugandan President: Being gay not a right

Gay Ugandans committing suicide

"It's the lack of empathy that makes it very easy for us to plunge into wars," the President said afterwards.

Presumably he was referring to military wars though that sentiment is obviously applicable to the culture war as well.

For only a lack of empathy could prompt officials at a Christian school in Virginia to suggest an 8-year-old girl attend a different school because she wasn't behaving in a feminine enough way to stay at theirs. (The school says it can't provide details because of confidentiality but "this matter is far beyond a simple 'hairstyle and tomboy issue' as inaccurately portrayed.")

Only a lack of empathy could lead Catholic school officials to fire a beloved teacher for planning to marry the love of his life. A lack of empathy is the reason why "The "700 Club's" Pat Robertson can be flippant about stoning gay people to death in the days of Jesus or lead Franklin Graham -- Billy Graham's son -- to suggest that LGBT people might adopt children in order to "recruit."

All of these incidents occurred after Pope Francis asked "who am I to judge?" as if ostracizing an elementary school tomboy was the best answer some of the Christian faith had to that question.

On the day the story of the little girl made national headlines, Dan Haseltine, lead singer for the Christian band Jars of Clay, tweeted, "why do I find myself regularly shaking my head, or rolling my eyes at supposed Christian leaders." He followed with, "I am learning that love's most potent manifestation happens when we make room at the table for everyone."

Love's most potent manifestation happens when we make room at the table for everyone.

Dan Haseltine

Haseltine's remarks were refreshing because many Christians in his position prefer silent disapproval in response to over-the-top quotes and head-scratching stories like the one involving the little girl. If you're curious why more and more people are leaving the church maybe it's because they hear "love the sinner but hate the sin" from the pulpit -- and then see so many followers go about their day as if simply saying the word "love" excused them from actually having to show any.

Many followers of Christ may find silence on this topic easier, not noticing silence is being interpreted as endorsements -- whether they agree with the anti-gay rhetoric or not. For each time "Who am I to judge?" is asked, an anti-gay quote from the Catholic League's Bill Donahue is there to answer.

To move Pope Francis' question from a global headline to global change, Christians must stop allowing silence to be the de facto weapon of choice against the senseless persecution of gay people.

Forty percent of the country's homeless youth identify as LGBT, with nearly half kicked out of their homes by those who are supposed to love and care for them. There's a sermon topic we don't hear every week -- but we should.

Slave owners used to start and end each service on the plantation with Ephesians 6:5 -- "Servants, be obedient to them that are your masters according to the flesh, with fear and trembling, in singleness of your heart, as unto Christ."

"Who am I to judge?" was not enough to change that.

Women were once compelled to stay with abusive husbands with the words "wives, submit yourselves unto your own husbands, as unto the Lord."

"Who am I to judge?" was not enough to empower women and get the legal system to prosecute their assailants.

It was in such silence that laws in Uganda were originally drafted to make being gay punishable by death. Laws that found validation from the ties some Ugandan politicians had to well known U.S. evangelicals who regularly visited the country. The evangelicals have since distanced themselves from the anti-gay politics of Uganda. But not because of some moral stirring within their soul but because they were called out on it.

Because the silence ended.

Cardinal Timothy Dolan -- when asked about NFL prospect Michael Sam coming out of the closet -- said on NBC's "Meet The Press," "The same Bible that tells us that teaches us well about the virtues of chastity and the virtue of fidelity and marriage also tells us not to judge people."

A truism that is ignored way too often by anti-gay Christian leaders. A truism that isn't reiterated enough by those who follow them.

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Spain: Not enough refugees receiving asylum

Half of all immigrants who jump the fence in Ceuta and Melilla are refugees but only 10% of them manage to receive asylum.

According to a report by the Spanish Commission for Refugee Aid (CEAR) around half of the immigrants who jump the fences come from war-torn countries and should be given asylum in Spain although only around 10% of them receive it.

CEAR has presented a campaign to collect 100,000 signatures with which to ask the government to take away the concertinaed barbed wire from the top of the fence and to respect human rights.

CEAR explains that refugees “are people who never expected to have to leave their countries and ask for asylum in order to preserve their lives and physical safety.”

Carlos Berzosa, president of CEAR, has claims that the government is trying to make out that jumping the fence is a recent problem in order to justify its “repressive methods,” when in actual fact this has always happened at that border.

He goes on to say that only 5% of illegal immigrants come in through the fences in Ceuta and Melilla - the rest come in through airports, roads and trains.

Spain: Divorce rates are dropping

Divorce and separation rates continue to drop while the rate of annulments is on the rise.

According to a press release by the CGPJ (General Judiciary Council) in 2013 there were 1.7% less divorces and separations but 8.5% more annulments.

Unfortunately for the overloaded court system the ancillary cases that accompany annulments and divorces are the same; these include custody battles and alimony cases.

This drop in divorces, both by mutual agreement and non-consensual, has been attributed to, among other things, the expense involved at a time of recession.

How the Big Bang finding came about

This long-exposure image from the Hubble Telescope is the deepest-ever picture taken of a cluster of galaxies. The cluster, called Abell 2744, contains several hundred galaxies as they looked 3.5 billion years ago; the more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang.

NASA's NuSTAR telescope array generated the first map of radioactivity in the remnants of an exploding star, or supernova. Blue in this image of Cassiopeia A represents radioactive material. Click through to see other wonders of the universe.

A supernova was spotted on January 21 in Messier 82, one of the nearest big galaxies. This wide view image was taken on January 22.

The M82 supernova, seen here, has been designated SN2014J because it is the 10th supernova detected in 2014. At 11.4 million light years from Earth, it is the closest Type Ia supernova recorded since systematic studies with telescopes began in the 1930s.

Is that a giant hand waving at us? Actually, it's what's left of a star that died and exploded a long time ago. Astronomers nicknamed it the "Hand of God." NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, took this image in high-energy X-rays, shown in blue. The image was combined with images from another space telescope, the Chandra X-ray Observatory.

The Hubble Space Telescope captured this image of the Southern Pinwheel Galaxy, one of the largest and closest galaxies of its kind. The center of the galaxy is mysterious, researchers say, because it has a double nucleus -- a supermassive black hole that may be ringed by a lopsided disc of stars, giving it the appearance of a dual core.

Hubble scientists say this is the best-ever view of the Tarantula Nebula, which is located in one of our closest galactic neighbors, the Large Magellanic Cloud.

Those spots on our sun appear small, but even a moderate-sized spot is about as big as Earth. They occur when strong magnetic fields poke through the sun's surface and let the area cool in comparison to the surrounding area.

This Hubble image looks a floating marble or a maybe a giant, disembodied eye. But it's actually a nebula with a giant star at its center. Scientists think the star used to be 20 times more massive than our sun, but it's dying and is destined to go supernova.

• Meg Urry describes the sequence of discoveries that lead to evidence of "inflation"


• Urry: Inflation, far faster than speed of light, happened in the first instant of the universe


• Urry: Theory began 80 years ago with Edwin Hubble: Telescope named after him


• Urry: After inflation, the universe went into more "sedate" pace of Big Bang we see now

Editor's note: Meg Urry is the Israel Munson professor of physics and astronomy at Yale University and director of the Yale Center for Astronomy and Astrophysics.

(CNN) -- For the past week-and-a-half, people have been marveling over the discovery of evidence supporting "inflation," the theory describing the birth pangs of the Big Bang 13.7 billions years ago. What do these findings mean and how did they come about?

Lots of articles reported the news, but I am going to try to explain it in depth. Stick with me, because this is one of the most exciting astrophysical discoveries in decades.

Humans have wondered about the origin of the universe for millennia, and last week's news brought us a little closer to an answer. What this development means, basically, is that for the first time, we may be seeing what happened in the first billionth of a trillionth of a trillionth of a second of the universe.

Assuming this discovery is verified by other similar experiments, it means the very birth of the universe can be studied. These will tell us about the physics of matter and energy well beyond the reach of earthly particle accelerators like the Large Hadron Collider.

In a press conference on March 17, leaders of the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) experiment announced their discovery of evidence of gravitational waves -- predicted by Einstein's theory of General Relativity -- that were generated by the near-instantaneous expansion of the universe by some 50 factors of 10, or a factor of 100 million, trillion, trillion. Those waves were predicted by the theory of inflation, developed 30 years ago by Alan Guth, Andrei Linde and others.

Inflation is the instantaneous initial expansion, far faster than the speed of light, that "describes the propulsion mechanism that drove the universe into the period of tremendous expansion that we call the Big Bang," as Alan Guth put it. Incidentally, the term "Big Bang" was coined as an insult by a physicist who didn't like the theory.

The Big Bang idea itself is simple. Edwin Hubble -- after whom the Hubble Space Telescope is named -- showed more than 80 years ago that our universe is expanding. Objects in space are not hurtling outward: Space itself is becoming bigger over time. That means the distance between two galaxies grows even if neither galaxy is moving through space at all.

By extrapolating the Hubble expansion backward, we have long known that the universe was once smaller by many, many factors of 10. All the mass and energy of the entire universe squeezed into such a tiny volume would have been much hotter and denser. Then, as the universe expanded over time, the energy density went down, so the temperature cooled. This Big Bang idea implied that cool relic radiation should be visible today.

Indeed, this Big Bang glow of radiation was discovered in the early 1960s by two Bell Labs engineers, Arno Penzias and Robert Wilson, who were trying to build the world's best radio antenna.

Ripples in space-time revealed

A Big Bang breakthrough?

Their instrument recorded a mysterious irreducible low-level noise from every direction. Apparently worried that the surface of the antenna horn had been corrupted by, um, debris from pigeons roosting inside, the engineers repeatedly disassembled and cleaned the antenna, to no avail.

Physicists later connected this measurement to the Big Bang prediction of a cosmic microwave background, for which Penzias and Wilson were awarded the Nobel Prize in 1978. As a colleague at Bell Labs joked, referring to their obsession with pigeon droppings, "They went looking for dung and came up with gold. For most of us [scientists], it works the other way."

The Big Bang idea was well established by the 1980s. But it did not explain important pieces of the story.

First, the universe looks pretty homogeneous and isotropic -- that is, galaxies in any one direction look very similar to galaxies in the opposite direction, no matter how distant. The number of galaxies, their masses, their shapes and their stellar content are remarkably similar, to the furthest reaches we can observe.

This is surprising because the Big Bang-Hubble expansion implied that very distant regions should never have been in causal contact. How then could they be so similar? Here is a simple analogy: Imagine a thermos of ice water and a thermos of hot tea. As long as these two liquids are separate, they will have different temperatures. But if the two liquids are combined, the mixed liquid will quickly reach an intermediate temperature. Similarly, two well-separated regions of the universe can be alike only if they were at one time in contact.

The theory of inflation explains this quite naturally: If at the beginning the universe inflated at an extraordinarily rapid rate -- much faster than the Hubble-measured expansion today -- then all parts of the universe visible today were once in contact. That means they had the same initial physical conditions (such as temperature and density), so that similar stars and galaxies eventually formed out of the cosmic soup.

Inflation also explains why the universe has a very "flat" geometry -- something revealed in the 1990s by analysis of the spatial distribution of tiny fluctuations (hot and cold spots) in the cosmic microwave background radiation.

In principle, other geometries of space were possible. For example, a two-dimensional surface can be flat like a table; convex like the surface of a sphere (also called open); or concave like the surface of a saddle (also called closed).

For the universe to be flat requires a very precise balance. It has infinitely more ways to be open or closed, with strong curvature, weak curvature, or anything in between. But to be flat -- well, that's like balancing on a knife edge. Inflation naturally explains this odd fact.

Specifically, the idea is that, at the very beginning, the universe must have inflated enough to stretch the fabric of space until no trace of curvature remained. Imagine inflating a beach ball to the size of the Earth: you can easily see the curvature of the beach ball in your hands but once it's hyper-inflated, any piece of its surface seems very flat, just as the Earth feels flat locally.

The enormous inflation in size would effectively erase the initial conditions in the universe. Whatever the initial temperature, for example, inflation would cool the universe to absolute zero. Even if the initial universe were very lumpy, after inflation we can see only a very smooth, local part of the original volume -- and it would seem perfectly flat.

After about one hundred millionth of a trillionth of a trillionth of a second, according to theorists, whatever repulsive gravity caused inflation then transformed into a hot, dense soup of particles and energy. At this point, the Big Bang expansion that Hubble discovered took over.

How inflation began and how it ended are not yet understood, but this simple idea of inflation neatly explains otherwise odd characteristics -- isotropy and flatness -- of our universe.

Still, until now, there had been no direct evidence of inflation. What BICEP2 saw was the imprint of inflation on the cosmic microwave background radiation.

Specifically, inflation should have generated a lot of gravitational waves -- that is, it would cause propagating ripples of space itself. Such waves have a characteristic pattern, squeezing space rhythmically in one direction then the perpendicular direction, like two hands pressing a rubber ball top to bottom then side to side.

This distortion of space causes a special pattern of polarization in the Cosmic Microwave Background radiation. So what is polarization?

Light is a wave that oscillates back and forth -- polarized light oscillates preferentially in one plane. Because most light is a mix of random directions of polarization, its net polarization is zero. But any scattered light, like sunlight reflected off water, is polarized -- which is why polarized sunglasses cut down substantially on glare.

BICEP2 scientists searched for that special pattern of polarization in the cosmic microwave background that would show the evidence of inflation, working for several years analyzing and reanalyzing their data.

As they ran through every possible check of the analysis, team members finally began to believe they had detected the first direct signs of inflation.

Now other experiment teams are redoubling efforts to find the same signal -- or to find contradictions. The reported BICEP2 signal is unexpectedly strong, so it should be within reach of at least some of these experiments.

Physicists around the world know: the BICEP2 discovery is only the beginning of the story. If this result is verified by independent experiments, new, more accurate experiments will be designed to better measure the polarization imprint. This in turn will tell us about how matter and energy behave in conditions much hotter and denser than on Earth or any other place in the cosmos.

As Carl Sagan once said, "Extraordinary claims require extraordinary evidence." Let the observations begin.

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The opinions expressed in this commentary are solely those of Meg Urry.