After the strike ended, the show was picked up for a second season, airing in the 2008–2009 season, premiering in the same time slot on September 22, 2008.[17] With increasing ratings, the show received a two-year renewal through the 2010–11 season in 2009.[18][19] In 2011, the show was picked up for three more seasons.[20] In March 2014, the show was renewed again for three more years through the 2016–17 season. This marked the second time the series gained a three-year renewal.[21] In March 2017, the series was renewed for two additional seasons, bringing its total to 12, and running through the 2018–19 television season.[22]
Howard dresses as Sheldon for Halloween at work, hurting Sheldon's feelings. When Howard does not apologize, Amy and Sheldon dress as Bernadette and Howard at Penny and Leonard's party, angering Bernadette. Sheldon and Bernadette later bond over their painful childhood memories. Leonard is upset that Penny does not seem to remember that their first kiss was on Halloween. Penny later admits she does remember, but does not like to think of their first kiss as an unromantic drunken hookup. She likes to imagine their first kiss was one they shared on Leonard's birthday, which pleases him.
Sheldon and Amy's paper is a huge success, but Sheldon is upset when he is not allowed to do interviews, while Amy gets all the attention. Denise asks Stuart to move in with her but he panics and flees. Raj sets up a doorbell camera app for Anu but sees her greet her ex-boyfriend. At a paintball match, all the feuding couples argue, with Raj saying he cannot trust Anu since he does not even know her, throwing the wedding into question. Stuart makes up with Denise by giving her a key to the Wolowitz house. Sheldon tries to do one interview with Amy but leaves when he is asked about the Russian scientists who had earlier disproved their theory.
Regina King reprised her role as Janine Davis in “The Retraction Reaction”. Bob Newhart returned as Arthur Jeffries in “The Proton Regeneration”. Dean Norris reprised his role as Colonel Williams in “The Tesla Recoil”. Brian Thomas Smith returned as Zack in “The Bitcoin Entanglement”. Brian George guest starred as Raj’s father, Dr. Koothrappali, in “The Confidence Erosion”. Joshua Malina reprised his role as President Siebert in “The Monetary Insufficiency”.
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Sheldon's mother Mary refuses to attend his wedding unless he invites his older brother George, with whom he is no longer speaking. Sheldon and Leonard fly to Texas to talk to George, who now owns a chain of tire stores. He refuses to attend and later explains to Leonard and Sheldon that the family sacrificed a lot to fund Sheldon's education, and he was stuck caring for their mother and sister Missy after their father died while Sheldon was in college. Sheldon never thanked him for this. Leonard takes George back to the hotel room where Sheldon apologizes for what he did and the brothers reconcile. George agrees to attend the wedding. Meanwhile, Howard and Bernadette's children contract pinkeye, infecting everyone except Penny, much to Amy's fury.
Cosmologist Stephen Hawking made a short guest appearance in the fifth-season episode;[57] in the eighth season, Hawking video conferences with Sheldon and Leonard, and makes another appearance in the 200th episode. In the fifth and sixth seasons, NASA astronaut Michael J. Massimino played himself multiple times in the role of Howard's fellow astronaut. Bill Nye appeared in the seventh and twelfth seasons.
Science has also interfered with the characters' romantic lives. Leslie breaks up with Leonard when he sides with Sheldon in his support for string theory rather than loop quantum gravity.[70] When Leonard joins Sheldon, Raj, and Howard on a three-month Arctic research trip, it separates Leonard and Penny at a time when their relationship is budding. When Bernadette takes an interest in Leonard's work, it makes both Penny and Howard envious and results in Howard confronting Leonard, and Penny asking Sheldon to teach her physics.[71] Sheldon and Amy also briefly end their relationship after an argument over which of their fields is superior.[72]
The fourth season's second half began on June 30, 2011. Season 5 began airing on November 3, 2011, at 8:00 p.m. as part of E4's Comedy Thursdays, acting as a lead-in to the channel's newest comedy, Perfect Couples. Episode 19, the highest-viewed episode of the season, attracted 1.4 million viewers.[145] Season 6 premiered on November 15, 2012, with 1.89 million viewers and a further 469,000 on the time shift channel, bringing the total to 2.31 million, E4's highest viewing ratings of 2012, and the highest the channel had received since June 2011. The sixth season returned in mid-2013 to finish airing the remaining episodes.[146] Season 7 premiered on E4 on October 31, 2013 at 8:30pm and hit multiple ratings records this season. The second half of season seven aired in mid 2014.[147] The eighth season premiered on E4 on October 23, 2014 at 8:30 p.m.[148] During its eighth season, The Big Bang Theory shared its 8:30 p.m. time period with fellow CBS comedy, 2 Broke Girls. Following the airing of the first eight episodes of that show's fourth season, The Big Bang Theory returned to finish airing its eighth season on March 19, 2015.[149]
Filming proceeds smoothly – makeup artists do touch-ups, a prop master replenishes the pickle supply – with humor and camaraderie between takes. When a reshoot is needed because Simon Helberg, who plays Howard, held his napkin in the wrong hand, Kaley Cuoco, who plays Penny, lifts her food box high and mockingly asks, “Does it go over my head? I don’t remember.”
Sheldon and Amy decide on a wedding date of May 12th. They want to have their reception at the Athenaeum at Caltech. While scouting the location, they are shocked to see Leonard and Penny eating there, as Leonard had told Sheldon their application had been denied. In reality, Leonard just wanted a place to avoid Sheldon. Kripke has booked the venue for his birthday party on the same day as the wedding. Leonard cleans radioactive sludge for Kripke to get the date for Sheldon as an apology for lying. Kripke eventually agrees to change the day but when Amy hears he wants to sing at the wedding, she suggests they get married at the Griffith Observatory instead. Meanwhile, Bernadette and Howard are unsure if one of them should quit their job to stay home with the children. Howard volunteers, but when left alone with the children he passes out exhausted, leaving Raj in charge. When Bernadette meets Penny for lunch, she realizes that she loves being at the office again, but doesn't want to leave the children. She and Howard promise to figure it out later after they take a nap.
In March 2017, CBS renewed the series for two additional seasons, bringing its total to twelve, and running through the 2018–19 television season.[3] On August 22, 2018, CBS and Warner Bros. Television officially announced that the twelfth season would be the series' last. This stems from Jim Parsons' decision to leave the series at the end of the season if the show were to have been renewed for a thirteenth season.[1][4][5] The series concluded with an hour-long finale consisting of two back-to-back episodes on May 16, 2019.[6][7] A retrospective, hosted by Johnny Galecki and Kaley Cuoco, aired at 9:30 P.M. ET/PT, the night of the finale, following the season two finale of Young Sheldon.[8]
Filming proceeds smoothly – makeup artists do touch-ups, a prop master replenishes the pickle supply – with humor and camaraderie between takes. When a reshoot is needed because Simon Helberg, who plays Howard, held his napkin in the wrong hand, Kaley Cuoco, who plays Penny, lifts her food box high and mockingly asks, “Does it go over my head? I don’t remember.”

“I have a very long and somewhat self-centered speech here, but I’d like to set it aside. Because this honor doesn’t just belong to me; I wouldn’t be up here if it weren’t for some very important people in my life,” he says, and thanks his family and his “other family.” “I was under a misapprehension that my accomplishments were mine alone. Nothing could be further from the truth. I have been encouraged, sustained, inspired, and tolerated, not only by my wife, but by the greatest group of friends anyone ever had.”

Star Trek in particular is frequently referenced and Sheldon identifies strongly with the character of Spock, so much so that when he is given a used napkin signed by Leonard Nimoy as a Christmas gift from Penny he is overwhelmed with excitement and gratitude ("I possess the DNA of Leonard Nimoy?!").[73] Star Trek: The Original Series cast member George Takei has made a cameo, and Leonard Nimoy made a cameo as the voice of Sheldon's vintage Mr. Spock action figure (both cameos were in dream sequences). Star Trek: The Next Generation cast members Brent Spiner and LeVar Burton have had cameos as themselves,[74][75] while Wil Wheaton has a recurring role as a fictionalized version of himself. Leonard and Sheldon have had conversations in the Klingon language.
The show made its United Kingdom debut on Channel 4 on February 14, 2008. The show was also shown as a 'first-look' on Channel 4's digital offshoot E4 prior to the main channel's airing. While the show's ratings were not deemed strong enough for the main channel, they were considered the opposite for E4. For each following season, all episodes were shown first-run on E4, with episodes only aired on the main channel in a repeat capacity, usually on a weekend morning. From the third season, the show aired in two parts, being split so that it could air new episodes for longer throughout the year. This was due to rising ratings. The first part began airing on December 17, 2009, at 9:00 p.m. while the second part, containing the remaining eleven episodes, began airing in the same time period from May 6, 2010. The first half of the fourth season began airing on November 4, 2010, at 9:00 p.m., drawing 877,000 viewers, with a further 256,000 watching on the E4+1 hour service. This gave the show an overall total of 1.13 million viewers, making it E4's most-watched programme for that week. The increased ratings continued over subsequent weeks.[144]
The Big Bang Theory started off slowly in the ratings, failing to make the top 50 in its first season (ranking 68th), and ranking 40th in its second season. When the third season premiered on September 21, 2009, however, The Big Bang Theory ranked as CBS's highest-rated show of that evening in the adults 18–49 demographic (4.6/10) along with a then-series-high 12.83 million viewers.[94] After the first three seasons aired at different times on Monday nights, CBS moved the show to Thursdays at 8:00 ET for the 2010–2011 schedule, to be in direct competition with NBC's Comedy Block and Fox's American Idol (then the longest reigning leading primetime show on U.S. television from 2004 to 2011).[95] During its fourth season, it became television's highest rated comedy, just barely beating out eight-year champ Two and a Half Men. However, in the age 18–49 demographic (the show's target age range), it was the second highest rated comedy, behind ABC's Modern Family. The fifth season opened with viewing figures of over 14 million.[96]

Raj hooks up with a recently separated woman, Nell, after one of his lectures at the planetarium. Her husband, Oliver, confronts Raj at the planetarium and then breaks down over the end of his marriage. After comforting Oliver, Raj talks to Nell and convinces her to give her husband another chance. Meanwhile, after diving once again into string theory and sprawling his work across his and Amy's apartment, Sheldon approaches a reluctant Leonard and Penny and convinces them to let him use his old room to study for a three-day trial period with an agreement to be quiet and stay out of their way. Sheldon surprisingly sticks to this agreement, being a reasonably good guest, which shockingly frustrates Leonard to no end. After Leonard lets out his frustrations, Sheldon then renegotiates the guest tenancy agreement for an extra few days and resumes being his normal demanding self, much to Leonard's relief.

He's since become attached to a handful of Netflix projects, appearing in their recent Ted Bundy film Extremely Wicked, Shockingly Evil and Vile, executive producing mentee Ryan O'Connell's autobiographical comedy series Special, and signing on to reprise his role in their movie version of Ryan Murphy's Broadway production of The Boys in the Band, due on the streaming service in 2020.

'The Big Bang Theory' executive producers Chuck Lorre, left, and Steve Hollland and stars Kevin Sussman, Mayim Bialik, Jim Parsons, Kaley Cuoco and Kunal Nayyar stand to the left of 'The Big Bang Theory' stage plaque, with executive producer Bill Prady, star Melissa Rauch, executive producer Steven Molaro and star Johnny Galecki, director Mark Cendrowski and star Simon Helberg on the right. (Photo: Warner Bros.)

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