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.
The show originally centered on five characters living in Pasadena, California: Leonard Hofstadter and Sheldon Cooper, both physicists at Caltech, who share an apartment; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's similarly geeky and socially awkward friends and co-workers, aerospace engineer Howard Wolowitz and astrophysicist Raj Koothrappali. Over time, supporting characters were promoted to starring roles, including neuroscientist Amy Farrah Fowler, microbiologist Bernadette Rostenkowski, physicist Leslie Winkle and comic book store owner Stuart Bloom.
When Raj misses out on a job at the planetarium, his father says he has no confidence because Howard always makes fun of him. Raj distances himself from Howard and later gets the job. Howard is hurt by this (even after apologizing to him) and tries to make amends with Raj after his first performance at the planetarium but, when he sees him get a date, he leaves without speaking to him, thinking that the latter's life is better without him. Meanwhile, Sheldon and Amy try randomly dividing up wedding tasks, but they keep arguing. The only thing they can agree on is to get married, so they decide to just get married at City Hall. However, Sheldon decides that he wants a first dance with his new wife and they go home to plan their wedding.
Sheldon has been writing to the reclusive scientist Dr. Wolcott (Peter MacNicol) who invites Sheldon to visit him in his completely isolated mountain cabin. The women quickly ask Leonard, Raj and Howard to tag along with him, making this his bachelor party, despite Sheldon's assertions that it shouldn't be called such. Wolcott has a brilliant mind but his behavior is very strange to the guys. He tells Sheldon he cut off all contact with others to focus on science. Though Sheldon enjoys the science, he decides that he doesn't want to live like Wolcott because of his friends and Amy. The guys leave to avoid Wolcott, whose theories about time make him doubt whether the guys were ever there. Meanwhile, Penny and Bernadette throw Amy a quilting bee for her bachelorette party, which she quickly finds boring. When Amy tells them she wants a night full of bad decisions as she earlier implied, they go to a bar where Amy drinks a few shots and passes out in twelve minutes. When she wakes up back at the apartment hungover and disappointed, the women lie and reassure her she had a wild time, involving her drunkenly riverdancing.
Howard and Bernadette's new neighbor Andy begins using floodlights mounted on his balcony that encroaches on their rear deck and hot tub. Investigating whether the lights comply with city code, they enlist Sheldon to help them with the paperwork, but Sheldon finds out Howard and Bernadette's own deck is also not code-compliant. He considers turning them in, but ultimately chooses friendship over the rules. He does report Andy though for violating property lines, allowing Howard and Bernadette to enjoy their deck again. Meanwhile, Bert wants to cut into a meteorite with Raj's help, rejecting Leonard's idea to use his laser to do so. Leonard dreams that he opens the meteorite with his laser, becomes possessed by an alien intelligence and eats Bert, Raj and Penny. He later apologizes to Raj and Bert, admitting he was jealous. The possibility of the meteorite unleashing a plague scares Stuart and Denise, who use it as foreplay for procreation, Stuart closing the store on Sheldon when he tries to browse.
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.
Initial reception for the series was mixed. The review aggregation website Rotten Tomatoes reported a 52% approval rating for the first season based on reviews from 23 critics, with an average rating of 5.18/10. The website's critical consensus reads, "The Big Bang Theory brings a new class of character to mainstream television, but much of the comedy feels formulaic and stiff." On Metacritic, the season holds a score of 57 out of 100, based on reviews from 23 critics, indicating "mixed or average reviews". Later seasons received more acclaim and in 2013, TV Guide ranked the series #52 on its list of the 60 Best Series of All Time.
Mayim Bialik as Amy Farrah Fowler (guest star season 3, starring seasons 4–12): A woman selected by an online dating site as Sheldon's perfect mate. Amy is from Glendale, CA. While she and Sheldon initially share social cluelessness, after befriending Penny and Bernadette she eventually becomes more interested in social and romantic interaction. Her relationship with Sheldon slowly progresses to the point at which Sheldon considers her his girlfriend, eventually they get married. Amy believes she and Penny are best friends, a sentiment that Penny does not initially share. Amy has a Ph.D. in neurobiology.
The Canadian alternative rock band Barenaked Ladies wrote and recorded the show's theme song, which describes the history and formation of the universe and the Earth. Co-lead singer Ed Robertson was asked by Lorre and Prady to write a theme song for the show after the producers attended one of the band's concerts in Los Angeles. By coincidence, Robertson had recently read Simon Singh's book Big Bang, and at the concert improvised a freestyle rap about the origins of the universe. Lorre and Prady phoned him shortly thereafter and asked him to write the theme song. Having been asked to write songs for other films and shows, but ending up being rejected because producers favored songs by other artists, Robertson agreed to write the theme only after learning that Lorre and Prady had not asked anyone else.
The show's pilot episode premiered on September 24, 2007. This was the second pilot produced for the show. A different pilot was produced for the 2006–07 television season but never aired. The structure of the original unaired pilot was substantially different from the series' current form. The only main characters retained in both pilots were Leonard (Johnny Galecki) and Sheldon (Jim Parsons), who are named after Sheldon Leonard, a longtime figure in episodic television as producer, director and actor. A minor character, Althea (Vernee Watson), appeared in the first scene of both pilots that was retained generally as-is. The first pilot included two female lead characters - Katie, "a street-hardened, tough-as-nails woman with a vulnerable interior" (played by Canadian actress Amanda Walsh) and Gilda, a scientist colleague and friend of the male characters (played by Iris Bahr). Sheldon and Leonard meet Katie after she breaks up with a boyfriend and they invite her to share their apartment. Gilda is threatened by Katie's presence. Test audiences reacted negatively to Katie, but they liked Sheldon and Leonard. The original pilot used Thomas Dolby's hit "She Blinded Me with Science" as its theme song.
“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.”
Sheldon and Amy are surprised to find that Wil Wheaton's new Professor Proton show is actually very entertaining and that Wil had Howard on as a guest. On Howard's suggestion, Sheldon apologizes to Wil, mending their friendship, and tells him he wants to be on the show; but Wil asks for Amy. Sheldon tells her to do it, but she says she often doesn't do things to avoid upsetting him. Sheldon is horrified when he realises how selfish he has been, especially when he finds out that the men do the same for him. Sheldon encourages Amy to go on the show to inspire girls to pursue science while trying to control his obvious jealousy for her. Meanwhile, Leonard writes a book about a physicist that solves a murder, the protagonist Logan Dean is based on himself. Bernadette points out Logan's mean boss Illsa is similar to Penny, who thinks it is based on Bernadette, though Leonard does not correct her. However, Bernadette tells Penny the truth, upsetting her and making her mad at Leonard. After calling his mother for advice, Leonard learns that Illsa is actually like her; but abandons the book when he realizes he wrote romantic tension into the relationship between the characters.