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. 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). 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.
Through the use of his vanity cards at the end of episodes, Lorre alleged that the program had been plagiarized by a show produced and aired in Belarus. Officially titled Теоретики (The Theorists), the show features "clones" of the main characters, a similar opening sequence, and what appears to be a very close Russian translation of the scripts. Lorre expressed annoyance and described his inquiry with the Warner Bros. legal department about options. The television production company and station's close relationship with the Belarus government was cited as the reason that any attempt to claim copyright infringement would be in vain because the company copying the episodes is operated by the government.
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.
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 go on their honeymoon to New York City, starting with Legoland New York. Sheldon's insistence on scheduling their marital relations stresses Amy. He says he only does it to remind himself to be intimate with her. They compromise by letting him make a schedule without sharing it with her. Leonard and Penny discover Mr. Larry Fowler (Teller) in Sheldon and Amy's apartment hiding from his wife (Kathy Bates). Leonard notices parallels in his own marriage, which offends Penny. Leonard apologizes, and Penny convinces Mrs. Fowler to give her husband a break. When Mrs. Fowler wants to hang out with her regularly, Penny scares Larry out of the building. Raj instigates a Twitter feud with Neil deGrasse Tyson but is scared off when confronted over the phone. Neil then calls Bill Nye to scare him, too.
Producers, writers and others, including recurring guest star Wil Wheaton, joined the festivities on set and — hardly a spoiler alert — more laughing, crying and hugging ensued. Lorre embraced a happy Parsons, then moved on to each cast member as if it were a reception line. Galecki shared long, emotional hugs with his colleagues. After trying to keep their emotions under control for so long, it was finally time to just let go.
Kunal Nayyar as Rajesh Koothrappali: A particle astrophysicist originally from New Delhi, India. Raj is initially unable to talk to women unless he drinks alcohol. Raj also has very feminine tastes and often takes on a stereotypical female role in his friendship with Howard as well as in the group of four men. Raj later dates Lucy (Kate Micucci), who also suffers from social anxiety, but it eventually ends; he later speaks to Penny without alcohol and thus overcomes his selective mutism. He begins dating Emily and their relationship later becomes exclusive. Raj also has a Yorkshire Terrier named Cinnamon.
Jim Parsons as Sheldon Cooper: Originally from Galveston, Texas, Sheldon was a child prodigy with an eidetic memory who began college at the age of eleven, and earned a PhD at age sixteen. He is a theoretical physicist researching quantum mechanics and string theory, and despite his IQ of 187, he finds many routine aspects of social situations difficult to grasp. He is determined to have his own way, continually boasts of his intelligence, and has an extremely ritualized way of living. Despite these quirks, he begins a relationship with Amy Farrah Fowler, and they eventually marry.
When Leonard receives a braggy Christmas letter from his brother, he feels bad about how little he and Penny have achieved in the past year. Leonard wonders if he and Penny should be thinking about the next step after two years of marriage, like buying a house or having a baby. Penny tells him she wants to do all those things someday, but she has a bunch of stuff she would like to do first.
Amy and Howard begin collaborating on a neuroprosthetics project, upsetting Sheldon and Raj, who feel they are not spending as much time with them. When they seek solace with an annoyed Bernadette, she exploits the situation to trick Sheldon into doing Howard's chores. Penny successfully employs the approaches recommended in one of Bernadette's parenting books to deal with Sheldon, but Leonard believes she is coddling Sheldon too much. Sheldon and Raj resolve the issue of missing their significant others by spending time together.